Pull Requests

# On the server. Opening pull request 1 for feature/priority-routing, as Git data:
$ cd server/ticket-router.git
$ git update-ref refs/pull/1/head refs/heads/feature/priority-routing
# Clients may read refs/pull/ but not write it:
$ git config set receive.hideRefs refs/pull
$ git for-each-ref --format="%(objectname:short) %(refname)"
16d4788 refs/heads/feature/priority-routing
9aa221a refs/heads/main
16d4788 refs/pull/1/head

$ cd ../../asha/ticket-router
$ git ls-remote origin
9aa221a9fb05716040012e33f1e7c3074f948edc	HEAD
16d4788572b31e17e4c3104a1d9861a5ce2c47ea	refs/heads/feature/priority-routing
9aa221a9fb05716040012e33f1e7c3074f948edc	refs/heads/main
16d4788572b31e17e4c3104a1d9861a5ce2c47ea	refs/pull/1/head
# A clone fetches refs/heads/* only, so the pull request ref is not in Asha's clone:
$ git config get --all remote.origin.fetch
+refs/heads/*:refs/remotes/origin/*
$ git fetch
$ git for-each-ref --format="%(refname)" refs/remotes refs/pull
refs/remotes/origin/HEAD
refs/remotes/origin/feature/priority-routing
refs/remotes/origin/main

$ git fetch origin pull/1/head:pr-1
From ../../server/ticket-router
 * [new ref]         refs/pull/1/head -> pr-1
$ git log --oneline -3 pr-1
16d4788 Fix the name of the escalations queue
12ae95d Route high-priority tickets to escalation
44c1e7b Add priority scoring
$ git rev-parse pr-1 origin/feature/priority-routing
16d4788572b31e17e4c3104a1d9861a5ce2c47ea
16d4788572b31e17e4c3104a1d9861a5ce2c47ea

$ git push origin pr-1:refs/pull/1/head
To ../../server/ticket-router.git
 ! [remote rejected] pr-1 -> refs/pull/1/head (deny updating a hidden ref)
error: failed to push some refs to '../../server/ticket-router.git'
[exit status: 1]
$ git push origin main:refs/pull/9/head
To ../../server/ticket-router.git
 ! [remote rejected] main -> refs/pull/9/head (deny updating a hidden ref)
error: failed to push some refs to '../../server/ticket-router.git'
[exit status: 1]

# On the server. A merge without a working tree (Chapter 8, section 8.17):
$ cd ../../server/ticket-router.git
$ git merge-tree --write-tree main refs/pull/1/head
beff5a60b5d7f4a2b93e41130391fc485f5970e0
[exit status: 0]
$ tree=$(git merge-tree --write-tree main refs/pull/1/head)
$ merge=$(git commit-tree $tree -p main -p refs/pull/1/head -m "Merge refs/pull/1/head into main")
$ git update-ref refs/pull/1/merge $merge
$ git log --oneline --graph -5 refs/pull/1/merge
*   135aad1 Merge refs/pull/1/head into main
|\  
| * 16d4788 Fix the name of the escalations queue
| * 12ae95d Route high-priority tickets to escalation
| * 44c1e7b Add priority scoring
* | 9aa221a Raise the confidence threshold to 0.7
|/  
$ git for-each-ref --format="%(objectname:short) %(refname)" refs/heads refs/pull
16d4788 refs/heads/feature/priority-routing
9aa221a refs/heads/main
16d4788 refs/pull/1/head
135aad1 refs/pull/1/merge

# The test merge commit is on neither branch:
$ git branch --contains refs/pull/1/merge
$ git show -s --format="%h parents: %p" refs/pull/1/merge
135aad1 parents: 9aa221a 16d4788
$ git rev-parse main refs/pull/1/head
9aa221a9fb05716040012e33f1e7c3074f948edc
16d4788572b31e17e4c3104a1d9861a5ce2c47ea

  base repository on the server

  fbbcc8d---53e7f57---f3e7ca9---9a383e5---9aa221a              refs/heads/main
                                    \            \
                                     \            135aad1      refs/pull/1/merge   (test merge; on no branch)
                                      \          /
                                       44c1e7b---12ae95d---16d4788
                                                              |
                                                              +-- refs/heads/feature/priority-routing
                                                              +-- refs/pull/1/head   (read-only)

  GitHub's database:  pull request #1  { base: main, head: feature/priority-routing, title, reviews, checks ... }

# On the server, after one more commit landed on main:
$ git rev-parse --short main
028b1de
$ git show -s --format="%h parents: %p" refs/pull/1/merge
135aad1 parents: 9aa221a 16d4788
# The merge ref is a stored commit. It is as fresh as its first parent.
$ git merge-base --is-ancestor main refs/pull/1/merge
[exit status: 1]

# Pull request 2 changes a line that main has changed since. On the server:
$ git merge-tree --write-tree --name-only main refs/pull/2/head
25dcf7083388484e218d7dc1b73f8e560e6c0909
config/routing.yaml

Auto-merging config/routing.yaml
CONFLICT (content): Merge conflict in config/routing.yaml
[exit status: 1]
# No clean tree, so no test merge commit and no refs/pull/2/merge:
$ git for-each-ref --format="%(refname)" refs/pull
refs/pull/1/head
refs/pull/1/merge
refs/pull/2/head

# The head branch is deleted. The pull request ref still names the commits:
$ git update-ref -d refs/heads/feature/priority-routing
$ git for-each-ref --format="%(objectname:short) %(refname)" refs/heads refs/pull
e996b41 refs/heads/fix/threshold
d8c296f refs/heads/main
16d4788 refs/pull/1/head
135aad1 refs/pull/1/merge
e996b41 refs/pull/2/head
$ cd ../../ravi/ticket-router
$ git fetch --prune
From ../../server/ticket-router
 - [deleted]         (none)     -> origin/feature/priority-routing
   028b1de..d8c296f  main       -> origin/main
$ git fetch origin pull/1/head
From ../../server/ticket-router
 * branch            refs/pull/1/head -> FETCH_HEAD
$ git log --oneline -1 FETCH_HEAD
16d4788 Fix the name of the escalations queue

$ git fetch
From ../../server/ticket-router
   9a383e5..9aa221a  main       -> origin/main
$ git log --oneline --graph origin/main feature/priority-routing
* 9aa221a Raise the confidence threshold to 0.7
| * 16d4788 Fix the name of the escalations queue
| * 12ae95d Route high-priority tickets to escalation
| * 44c1e7b Add priority scoring
|/  
* 9a383e5 Add classifier test
* f3e7ca9 Add routing config
* 53e7f57 Add keyword classifier
* fbbcc8d Add README

# The "Commits" tab: commits reachable from the head branch and not from the base branch.
$ git log --oneline origin/main..feature/priority-routing
16d4788 Fix the name of the escalations queue
12ae95d Route high-priority tickets to escalation
44c1e7b Add priority scoring
$ git rev-list --count origin/main..feature/priority-routing
3

$ git merge-base origin/main feature/priority-routing
9a383e547a1c8840f3c5c7b23bfab6120f366ed0
$ git log --oneline -1 $(git merge-base origin/main feature/priority-routing)
9a383e5 Add classifier test

# The "Files changed" tab: merge base compared with the head. Three dots.
$ git diff --stat origin/main...feature/priority-routing
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

# Two dots compare the two tips. The newer commit on main appears, reversed.
$ git diff --stat origin/main..feature/priority-routing
 config/routing.yaml | 2 +-
 router/classify.py  | 6 +++++-
 router/priority.py  | 6 ++++++
 3 files changed, 12 insertions(+), 2 deletions(-)
$ git diff origin/main..feature/priority-routing -- config/routing.yaml
diff --git a/config/routing.yaml b/config/routing.yaml
index b97f8de..a84cfc9 100644
--- a/config/routing.yaml
+++ b/config/routing.yaml
@@ -1,3 +1,3 @@
 model: router-small-v1
-confidence_threshold: 0.7
+confidence_threshold: 0.6
 fallback_queue: general

# A three-dot diff is a two-dot diff whose left side is the merge base.
$ git diff --stat $(git merge-base origin/main feature/priority-routing) feature/priority-routing
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

                       9aa221a                 main            two-dot:   9aa221a  compared with  16d4788
                      /
  ...f3e7ca9---9a383e5                                         three-dot: 9a383e5  compared with  16d4788
                      \                                                   (merge base)
                       44c1e7b---12ae95d---16d4788             feature/priority-routing
                       \_______________________/
                        git log main..feature: the "Commits" tab

# After the base branch is merged into the head branch, the tip of main is the merge base.
$ git merge -q origin/main
$ git log --oneline -1 $(git merge-base origin/main feature/priority-routing)
9aa221a Raise the confidence threshold to 0.7
$ git diff --stat origin/main..feature/priority-routing
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
$ git diff --stat origin/main...feature/priority-routing
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
$ git log --oneline origin/main..feature/priority-routing
8335b40 Merge remote-tracking branch 'origin/main' into feature/priority-routing
16d4788 Fix the name of the escalations queue
12ae95d Route high-priority tickets to escalation
44c1e7b Add priority scoring

# Ravi. His pull request fix/threshold -> main, and the test merge:
$ git log --oneline origin/main..fix/threshold
0e18437 Document how the threshold was tuned
be88439 Lower the confidence threshold to 0.65
$ git merge-tree --write-tree --name-only origin/main fix/threshold
e7f43627eaf0524ac48167b60df524db4bde078c
config/routing.yaml

Auto-merging config/routing.yaml
CONFLICT (content): Merge conflict in config/routing.yaml
[exit status: 1]

# Repair 1: merge the base branch into the head branch.
$ git merge origin/main
Auto-merging config/routing.yaml
CONFLICT (content): Merge conflict in config/routing.yaml
Automatic merge failed; fix conflicts and then commit the result.
[exit status: 1]
$ git diff
diff --cc config/routing.yaml
index 338a664,b97f8de..0000000
--- a/config/routing.yaml
+++ b/config/routing.yaml
@@@ -1,3 -1,3 +1,7 @@@
  model: router-small-v1
++<<<<<<< HEAD
 +confidence_threshold: 0.65
++=======
+ confidence_threshold: 0.7
++>>>>>>> origin/main
  fallback_queue: general
$ printf 'model: router-small-v1\nconfidence_threshold: 0.65\nfallback_queue: general\n' > config/routing.yaml
$ git add config/routing.yaml
$ git commit -q -m "Merge main into fix/threshold"
$ git push
To ../../server/ticket-router.git
   0e18437..ce88db6  fix/threshold -> fix/threshold

$ git log --oneline --graph -5
*   ce88db6 Merge main into fix/threshold
|\  
| * 4b21efb Raise the confidence threshold to 0.7
* | 0e18437 Document how the threshold was tuned
* | be88439 Lower the confidence threshold to 0.65
|/  
* 9a383e5 Add classifier test
$ git log --oneline origin/main..fix/threshold
ce88db6 Merge main into fix/threshold
0e18437 Document how the threshold was tuned
be88439 Lower the confidence threshold to 0.65
$ git merge-tree --write-tree origin/main fix/threshold
47f5aab81203b78e95a028e97e22c7935c94b122
[exit status: 0]

# Repair 2, in a copy of the clone: rebase the head branch onto the base.
$ cd ../../rebase-copy/ravi/ticket-router
$ git rebase origin/main
Rebasing (1/2)
Auto-merging config/routing.yaml
CONFLICT (content): Merge conflict in config/routing.yaml
error: could not apply be88439... Lower the confidence threshold to 0.65
hint: Resolve all conflicts manually, mark them as resolved with
hint: "git add/rm <conflicted_files>", then run "git rebase --continue".
hint: You can instead skip this commit: run "git rebase --skip".
hint: To abort and get back to the state before "git rebase", run "git rebase --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Could not apply be88439... # Lower the confidence threshold to 0.65
[exit status: 1]
$ printf 'model: router-small-v1\nconfidence_threshold: 0.65\nfallback_queue: general\n' > config/routing.yaml
$ git add config/routing.yaml
$ git rebase --continue
[detached HEAD e2639c7] Lower the confidence threshold to 0.65
 1 file changed, 1 insertion(+), 1 deletion(-)
Rebasing (2/2)
Successfully rebased and updated refs/heads/fix/threshold.

$ git log --oneline --graph -5
* fd22cf1 Document how the threshold was tuned
* e2639c7 Lower the confidence threshold to 0.65
* 4b21efb Raise the confidence threshold to 0.7
* 9a383e5 Add classifier test
* f3e7ca9 Add routing config
$ git log --oneline origin/main..fix/threshold
fd22cf1 Document how the threshold was tuned
e2639c7 Lower the confidence threshold to 0.65
$ git push
To ../../server/ticket-router.git
 ! [rejected]        fix/threshold -> fix/threshold (non-fast-forward)
error: failed to push some refs to '../../server/ticket-router.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. If you want to integrate the remote changes,
hint: use 'git pull' before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.
[exit status: 1]
$ git push --force-with-lease
To ../../server/ticket-router.git
 + 0e18437...fd22cf1 fix/threshold -> fix/threshold (forced update)

# Both repairs give reviewers the same three-dot diff:
$ git diff origin/main...fix/threshold
diff --git a/README.md b/README.md
index 98a45e9..d8990f5 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,5 @@
 # ticket-router
 
 Sends each support ticket to the queue that can answer it.
+
+The threshold is tuned on the March ticket sample.
diff --git a/config/routing.yaml b/config/routing.yaml
index b97f8de..338a664 100644
--- a/config/routing.yaml
+++ b/config/routing.yaml
@@ -1,3 +1,3 @@
 model: router-small-v1
-confidence_threshold: 0.7
+confidence_threshold: 0.65
 fallback_queue: general
$ diff <(git -C ../../../ravi/ticket-router diff origin/main...fix/threshold) <(git diff origin/main...fix/threshold) && echo identical
identical

# The same starting point in all three copies. Asha is the maintainer.
$ cd merge/asha
$ git log --graph --format="%h %an: %s" main origin/feature/priority-routing
* 9aa221a Asha Rao: Raise the confidence threshold to 0.7
| * 16d4788 Lab User: Fix the name of the escalations queue
| * 12ae95d Lab User: Route high-priority tickets to escalation
| * 44c1e7b Lab User: Add priority scoring
|/  
* 9a383e5 Asha Rao: Add classifier test
* f3e7ca9 Asha Rao: Add routing config
* 53e7f57 Asha Rao: Add keyword classifier
* fbbcc8d Asha Rao: Add README

# Method 1, "Create a merge commit":
$ git merge --no-ff -m "Merge pull request #1 from feature/priority-routing" origin/feature/priority-routing
Merge made by the 'ort' strategy.
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 router/priority.py
$ git push -q origin main
$ git log --graph --format="%h %an / %cn: %s" -6
*   4c53f2e Asha Rao / Asha Rao: Merge pull request #1 from feature/priority-routing
|\  
| * 16d4788 Lab User / Lab User: Fix the name of the escalations queue
| * 12ae95d Lab User / Lab User: Route high-priority tickets to escalation
| * 44c1e7b Lab User / Lab User: Add priority scoring
* | 9aa221a Asha Rao / Asha Rao: Raise the confidence threshold to 0.7
|/  
* 9a383e5 Asha Rao / Asha Rao: Add classifier test

# Method 2, "Squash and merge":
$ cd ../../squash/asha
$ git merge --squash origin/feature/priority-routing
Automatic merge went well; stopped before committing as requested
Squash commit -- not updating HEAD
$ git commit -q -m "Route high-priority tickets to an escalations queue (#1)"
$ git push -q origin main
$ git log --graph --format="%h %an / %cn: %s" -3
* da48bba Asha Rao / Asha Rao: Route high-priority tickets to an escalations queue (#1)
* 9aa221a Asha Rao / Asha Rao: Raise the confidence threshold to 0.7
* 9a383e5 Asha Rao / Asha Rao: Add classifier test
$ git show --stat --format="%h parents: %p" HEAD
da48bba parents: 9aa221a

 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

# Method 3, "Rebase and merge":
$ cd ../../rebase/asha
$ git switch -q -c pr-1 origin/feature/priority-routing
$ git rebase main
Rebasing (1/3)
Rebasing (2/3)
Rebasing (3/3)
Successfully rebased and updated refs/heads/pr-1.
$ git switch -q main
$ git merge --ff-only pr-1
Updating 9aa221a..ec49c9c
Fast-forward
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 router/priority.py
$ git push -q origin main
$ git log --graph --format="%h %an / %cn: %s" -5
* ec49c9c Lab User / Asha Rao: Fix the name of the escalations queue
* 2303f3a Lab User / Asha Rao: Route high-priority tickets to escalation
* dbb1f36 Lab User / Asha Rao: Add priority scoring
* 9aa221a Asha Rao / Asha Rao: Raise the confidence threshold to 0.7
* 9a383e5 Asha Rao / Asha Rao: Add classifier test

# The feature commits as you wrote them:
$ git -C merge/you log --format="%h author=%an committer=%cn %s" main..feature/priority-routing
16d4788 author=Lab User committer=Lab User Fix the name of the escalations queue
12ae95d author=Lab User committer=Lab User Route high-priority tickets to escalation
44c1e7b author=Lab User committer=Lab User Add priority scoring
# What main gained in each copy (main@{1} is where main was before):
$ git -C merge/asha log --format="%h author=%an committer=%cn %s" main@{1}..main
4c53f2e author=Asha Rao committer=Asha Rao Merge pull request #1 from feature/priority-routing
16d4788 author=Lab User committer=Lab User Fix the name of the escalations queue
12ae95d author=Lab User committer=Lab User Route high-priority tickets to escalation
44c1e7b author=Lab User committer=Lab User Add priority scoring
$ git -C squash/asha log --format="%h author=%an committer=%cn %s" main@{1}..main
da48bba author=Asha Rao committer=Asha Rao Route high-priority tickets to an escalations queue (#1)
$ git -C rebase/asha log --format="%h author=%an committer=%cn %s" main@{1}..main
ec49c9c author=Lab User committer=Asha Rao Fix the name of the escalations queue
2303f3a author=Lab User committer=Asha Rao Route high-priority tickets to escalation
dbb1f36 author=Lab User committer=Asha Rao Add priority scoring

# Three histories, one result: the tree of main is identical.
$ git -C merge/asha rev-parse main^{tree}
beff5a60b5d7f4a2b93e41130391fc485f5970e0
$ git -C squash/asha rev-parse main^{tree}
beff5a60b5d7f4a2b93e41130391fc485f5970e0
$ git -C rebase/asha rev-parse main^{tree}
beff5a60b5d7f4a2b93e41130391fc485f5970e0

  Create a merge commit                 Squash and merge              Rebase and merge

  ...9a383e5---9aa221a-------4c53f2e    ...9aa221a---da48bba          ...9aa221a---dbb1f36---2303f3a---ec49c9c
            \               /
             44c1e7b--12ae95d--16d4788   (44c1e7b, 12ae95d, 16d4788    (44c1e7b, 12ae95d, 16d4788
                                          stay on the old branch,       stay on the old branch,
                                          not on main)                  not on main)

  main keeps your IDs and adds one      main gets one new commit      main gets three new commits

# Asha, the maintainer. Nothing has landed on main since the branch was created:
$ git switch -q -c pr-1 origin/feature/priority-routing
$ git log --format="%h author=%an committer=%cn %s" main..pr-1
a4deec5 author=Lab User committer=Lab User Trigger CI again
9dcfb58 author=Lab User committer=Lab User Fix the name of the escalations queue
02820ea author=Lab User committer=Lab User Route high-priority tickets to escalation
3807b29 author=Lab User committer=Lab User Add priority scoring
$ git rebase main
Current branch pr-1 is up to date.
$ git log --format="%h author=%an committer=%cn %s" main..pr-1
a4deec5 author=Lab User committer=Lab User Trigger CI again
9dcfb58 author=Lab User committer=Lab User Fix the name of the escalations queue
02820ea author=Lab User committer=Lab User Route high-priority tickets to escalation
3807b29 author=Lab User committer=Lab User Add priority scoring

# What the documentation describes for the button: always new commits, new committer.
$ git rebase --no-ff main
Current branch pr-1 is up to date, rebase forced.
Rebasing (1/4)
Rebasing (2/4)
Rebasing (3/4)
Rebasing (4/4)
Successfully rebased and updated refs/heads/pr-1.
$ git log --format="%h author=%an committer=%cn %s" main..pr-1
123f252 author=Lab User committer=Asha Rao Trigger CI again
49f7387 author=Lab User committer=Asha Rao Fix the name of the escalations queue
9895c44 author=Lab User committer=Asha Rao Route high-priority tickets to escalation
216a2d1 author=Lab User committer=Asha Rao Add priority scoring

# And originally empty commits are dropped:
$ git rebase --no-ff --no-keep-empty main
Current branch pr-1 is up to date, rebase forced.
Rebasing (1/3)
Rebasing (2/3)
Rebasing (3/3)
Successfully rebased and updated refs/heads/pr-1.
$ git log --format="%h author=%an committer=%cn %s" main..pr-1
57537fc author=Lab User committer=Asha Rao Fix the name of the escalations queue
05bb2a6 author=Lab User committer=Asha Rao Route high-priority tickets to escalation
6da6459 author=Lab User committer=Asha Rao Add priority scoring

# Is your original head commit an ancestor of main?
$ git -C merge/asha merge-base --is-ancestor origin/feature/priority-routing main
[exit status: 0]
$ git -C squash/asha merge-base --is-ancestor origin/feature/priority-routing main
[exit status: 1]
$ git -C rebase/asha merge-base --is-ancestor origin/feature/priority-routing main
[exit status: 1]

# You, the contributor, after the merge. Copy 1 (merge commit):
$ cd merge/you
$ git pull --ff-only --prune
From ../server
 - [deleted]         (none)     -> origin/feature/priority-routing
   9aa221a..4c53f2e  main       -> origin/main
Updating 9aa221a..4c53f2e
Fast-forward
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 router/priority.py
$ git branch -vv
  feature/priority-routing 16d4788 [origin/feature/priority-routing: gone] Fix the name of the escalations queue
* main                     4c53f2e [origin/main] Merge pull request #1 from feature/priority-routing
$ git branch -d feature/priority-routing
Deleted branch feature/priority-routing (was 16d4788).
[exit status: 0]

# Copy 2 (squash):
$ cd ../../squash/you
$ git pull -q --ff-only --prune
$ git branch -vv
  feature/priority-routing 16d4788 [origin/feature/priority-routing: gone] Fix the name of the escalations queue
* main                     da48bba [origin/main] Route high-priority tickets to an escalations queue (#1)
$ git branch -d feature/priority-routing
error: the branch 'feature/priority-routing' is not fully merged
hint: If you are sure you want to delete it, run 'git branch -D feature/priority-routing'
hint: Disable this message with "git config set advice.forceDeleteBranch false"
[exit status: 1]

Observed behavior : After a squash merge (or a rebase merge) on GitHub, git branch -d refuses to
                    delete the local branch: "not fully merged".
Git state         : The branch tip 16d4788 is not an ancestor of main. main has da48bba, a commit
                    with one parent and the same content.
Mechanism         : git branch -d deletes only when the branch is merged into its upstream, or
                    into HEAD when it has no upstream. "Merged" means reachable. The upstream is
                    gone (pruned), so Git compares with HEAD and finds three unreachable commits.
Root cause        : Squash and rebase merges transfer content without ancestry.
Why Git does this : -d exists to stop you from deleting the only ref to commits. Git cannot know
                    that another commit carries the same changes.
Correct fix       : Verify that nothing would be lost, then delete with git branch -D.
Prevention        : None needed. Expect it under these two methods, and verify before -D.

# Rebase copy: every commit of the branch has an equivalent patch on main ("-"):
$ git cherry -v main feature/priority-routing
- 44c1e7b21c49dd09f0df26e7f5aa147834faba31 Add priority scoring
- 12ae95d534559f98f9aa705ed852f5a4700f6edd Route high-priority tickets to escalation
- 16d4788572b31e17e4c3104a1d9861a5ce2c47ea Fix the name of the escalations queue
# Squash copy: no single commit on main matches a commit of the branch ("+"):
$ cd ../../squash/you
$ git cherry -v main feature/priority-routing
+ 44c1e7b21c49dd09f0df26e7f5aa147834faba31 Add priority scoring
+ 12ae95d534559f98f9aa705ed852f5a4700f6edd Route high-priority tickets to escalation
+ 16d4788572b31e17e4c3104a1d9861a5ce2c47ea Fix the name of the escalations queue
# A test that works for both: would merging the branch change main at all?
$ git merge-tree --write-tree main feature/priority-routing
beff5a60b5d7f4a2b93e41130391fc485f5970e0
$ git rev-parse main^{tree}
beff5a60b5d7f4a2b93e41130391fc485f5970e0
$ git branch -D feature/priority-routing
Deleted branch feature/priority-routing (was 16d4788).

# Who does blame name for the new lines of classify.py?
$ git -C merge/asha blame -s -L 1,3 router/classify.py
12ae95d5 1) from router.priority import priority
12ae95d5 2) 
16d47885 3) QUEUES = ["billing", "technical", "general", "escalations"]
$ git -C squash/asha blame -s -L 1,3 router/classify.py
da48bba7 1) from router.priority import priority
da48bba7 2) 
da48bba7 3) QUEUES = ["billing", "technical", "general", "escalations"]
$ git -C rebase/asha blame -s -L 1,3 router/classify.py
2303f3af 1) from router.priority import priority
2303f3af 2) 
ec49c9cf 3) QUEUES = ["billing", "technical", "general", "escalations"]

# The second feature commit used the queue name "escalation"; the third one fixed it.
# Which commits on main contain the wrong name?
$ for c in $(git -C merge/asha rev-list main@{1}..main); do git -C merge/asha grep -c "\"escalation\"" $c -- router/classify.py; done
12ae95d534559f98f9aa705ed852f5a4700f6edd:router/classify.py:2
$ for c in $(git -C squash/asha rev-list main@{1}..main); do git -C squash/asha grep -c "\"escalation\"" $c -- router/classify.py; done
$ for c in $(git -C rebase/asha rev-list main@{1}..main); do git -C rebase/asha grep -c "\"escalation\"" $c -- router/classify.py; done
2303f3af71f7e0db4e5fbc21dcae7d7ba299cf72:router/classify.py:2

# Undoing the pull request on main. Copy 1: one revert, and you must name the mainline.
$ cd ../../merge/you
$ git revert --no-edit -m 1 HEAD
[main 4e8155e] Revert "Merge pull request #1 from feature/priority-routing"
 Date: Mon Sep 7 11:34:00 2026 +0530
 2 files changed, 1 insertion(+), 11 deletions(-)
 delete mode 100644 router/priority.py
$ git log --oneline -2
4e8155e Revert "Merge pull request #1 from feature/priority-routing"
4c53f2e Merge pull request #1 from feature/priority-routing

# Copy 2: one ordinary revert.
$ cd ../../squash/you
$ git revert --no-edit HEAD
[main 09f2610] Revert "Route high-priority tickets to an escalations queue (#1)"
 Date: Mon Sep 7 11:37:00 2026 +0530
 2 files changed, 1 insertion(+), 11 deletions(-)
 delete mode 100644 router/priority.py
$ git log --oneline -2
09f2610 Revert "Route high-priority tickets to an escalations queue (#1)"
da48bba Route high-priority tickets to an escalations queue (#1)

# Copy 3: one revert per commit, and you must know where the pull request began.
$ cd ../../rebase/you
$ git revert --no-edit HEAD~3..HEAD
[main d1270df] Revert "Fix the name of the escalations queue"
 Date: Mon Sep 7 11:40:00 2026 +0530
 1 file changed, 2 insertions(+), 2 deletions(-)
[main 7e41c05] Revert "Route high-priority tickets to escalation"
 Date: Mon Sep 7 11:40:00 2026 +0530
 1 file changed, 1 insertion(+), 5 deletions(-)
[main 7335fb5] Revert "Add priority scoring"
 Date: Mon Sep 7 11:40:00 2026 +0530
 1 file changed, 6 deletions(-)
 delete mode 100644 router/priority.py
$ git log --oneline -4
7335fb5 Revert "Add priority scoring"
7e41c05 Revert "Route high-priority tickets to escalation"
d1270df Revert "Fix the name of the escalations queue"
ec49c9c Fix the name of the escalations queue

gh repo edit OWNER/REPO --enable-auto-merge       # repository setting, once
gh pr merge 12 --auto --squash                    # merge when the requirements are met
gh pr merge 12 --disable-auto                     # withdraw the instruction

# The queue, in order 1, 2, 3. Each temporary branch contains main and everything ahead of it:
$ git switch -q -c gh-readonly-queue/main/pr-1 main
$ git merge -q --no-ff -m "Merge pull request #1" pr-1
$ git switch -q -c gh-readonly-queue/main/pr-2
$ git merge -q --no-ff -m "Merge pull request #2" pr-2
$ git switch -q -c gh-readonly-queue/main/pr-3
$ git merge -q --no-ff -m "Merge pull request #3" pr-3
$ git log --oneline --graph main..gh-readonly-queue/main/pr-3
*   0569767 Merge pull request #3
|\  
| * 6c162ef Describe how to run the tests
*   7c2fd05 Merge pull request #2
|\  
| * 1621dd9 Lower the confidence threshold to 0.65
* 2b79f02 Merge pull request #1
* 8c7493d Accept tickets without a subject

# The commits that the checks must test are none of the pull request heads:
$ git for-each-ref --format="%(objectname:short) %(refname:short)" refs/heads/pr-* refs/heads/gh-readonly-queue
2b79f02 gh-readonly-queue/main/pr-1
7c2fd05 gh-readonly-queue/main/pr-2
0569767 gh-readonly-queue/main/pr-3
8c7493d pr-1
1621dd9 pr-2
6c162ef pr-3

# The checks fail on the group of pull request 2. It leaves the queue; number 3 is rebuilt:
$ git branch -q -D gh-readonly-queue/main/pr-2
$ git switch -q -C gh-readonly-queue/main/pr-3 gh-readonly-queue/main/pr-1
$ git merge -q --no-ff -m "Merge pull request #3" pr-3
$ git log --oneline --graph main..gh-readonly-queue/main/pr-3
*   3bda42c Merge pull request #3
|\  
| * 6c162ef Describe how to run the tests
* 2b79f02 Merge pull request #1
* 8c7493d Accept tickets without a subject

# The checks pass. The base branch moves to the tested commit, unchanged:
$ git switch -q main
$ git merge --ff-only gh-readonly-queue/main/pr-3
Updating 9a383e5..3bda42c
Fast-forward
 README.md          | 2 ++
 router/classify.py | 2 +-
 2 files changed, 3 insertions(+), 1 deletion(-)
$ git log --oneline --first-parent -3
3bda42c Merge pull request #3
2b79f02 Merge pull request #1
9a383e5 Add classifier test

on:
  pull_request:
  merge_group:

# Pull request 1 was squash-merged. Its branch was not deleted.
$ git log --oneline --graph origin/main feature/priority-routing
* 1b2b8ed Route high-priority tickets to an escalations queue (#1)
* 9aa221a Raise the confidence threshold to 0.7
| * 16d4788 Fix the name of the escalations queue
| * 12ae95d Route high-priority tickets to escalation
| * 44c1e7b Add priority scoring
|/  
* 9a383e5 Add classifier test
* f3e7ca9 Add routing config
* 53e7f57 Add keyword classifier
* fbbcc8d Add README

# Pull request 2, same head branch, same base. Its commit list:
$ git log --oneline origin/main..feature/priority-routing
fa6e912 Treat data loss as urgent
16d4788 Fix the name of the escalations queue
12ae95d Route high-priority tickets to escalation
44c1e7b Add priority scoring
# Its diff (three dots), and what you believe you changed (your last commit):
$ git diff --stat origin/main...feature/priority-routing
 router/classify.py | 6 +++++-
 router/priority.py | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
$ git show --stat --format="%h %s" HEAD
fa6e912 Treat data loss as urgent

 router/priority.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

# The test merge of pull request 2:
$ git merge-tree --write-tree --name-only origin/main feature/priority-routing
55b98ddb581c6463837afef116da1f66dd495a8a
router/priority.py

Auto-merging router/priority.py
CONFLICT (add/add): Merge conflict in router/priority.py
[exit status: 1]

$ git log --oneline -1 $(git merge-base origin/main feature/priority-routing)
9a383e5 Add classifier test
$ git show -s --format="%h parents: %p  %s" origin/main
1b2b8ed parents: 9aa221a  Route high-priority tickets to an escalations queue (#1)

Observed behavior : Pull request 2 lists the three commits of pull request 1 again, shows their
                    changes again, and conflicts with main in router/priority.py.
Git state         : merge-base(main, branch) is still 9a383e5, the original fork point.
                    main's tip 1b2b8ed has one parent, 9aa221a.
Mechanism         : The squash commit copied the content of the branch and recorded no link to it.
                    For Git, 44c1e7b, 12ae95d and 16d4788 were never merged. base..head lists them.
                    The three-way merge sees "both sides added router/priority.py since 9a383e5,
                    with different content": an add/add conflict.
Root cause        : Content merged without ancestry (Chapter 8, section 8.12), then more work on top
                    of the old ancestry.
Why Git does this : The merge base comes from parent links only. Git never compares patches to
                    guess that a commit "is already there".
Correct fix       : Transplant the new commits onto main: git rebase --onto origin/main <last squashed commit>.
Prevention        : Delete the head branch after a squash or rebase merge and start new work from
                    main. Enable automatic deletion of head branches in the repository.

# Way out 1: move only the new commit onto main. Everything up to HEAD~1 is already there.
$ git rebase --onto origin/main HEAD~1
Rebasing (1/1)
Successfully rebased and updated refs/heads/feature/priority-routing.
$ git log --oneline origin/main..feature/priority-routing
e73426a Treat data loss as urgent
$ git diff --stat origin/main...feature/priority-routing
 router/priority.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git merge-tree --write-tree origin/main feature/priority-routing
084c79329c5ababb96ff69fb77466efba04e1ca0
[exit status: 0]
$ git push --force-with-lease
To ../../server/ticket-router.git
 + fa6e912...e73426a feature/priority-routing -> feature/priority-routing (forced update)

# In a copy of the clone: a plain rebase replays all four commits.
$ cd ../../you-plain-rebase/ticket-router
$ git rebase origin/main
Rebasing (1/4)
dropping 44c1e7b21c49dd09f0df26e7f5aa147834faba31 Add priority scoring -- patch contents already upstream
Rebasing (2/4)
Auto-merging router/classify.py
CONFLICT (content): Merge conflict in router/classify.py
error: could not apply 12ae95d... Route high-priority tickets to escalation
hint: Resolve all conflicts manually, mark them as resolved with
hint: "git add/rm <conflicted_files>", then run "git rebase --continue".
hint: You can instead skip this commit: run "git rebase --skip".
hint: To abort and get back to the state before "git rebase", run "git rebase --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Could not apply 12ae95d... # Route high-priority tickets to escalation
[exit status: 1]
$ git status --short
UU router/classify.py
$ git rebase --abort

# In another copy. Way out 2: merge main into the branch and resolve once.
$ cd ../../you-merge/ticket-router
$ git merge origin/main
Auto-merging router/priority.py
CONFLICT (add/add): Merge conflict in router/priority.py
Automatic merge failed; fix conflicts and then commit the result.
[exit status: 1]
# The branch has everything main has plus the new word, so the branch side is the answer:
$ git restore --ours router/priority.py
$ git add router/priority.py
$ git commit -q -m "Merge main into feature/priority-routing"
$ git log --oneline -1 $(git merge-base origin/main feature/priority-routing)
1b2b8ed Route high-priority tickets to an escalations queue (#1)
$ git diff --stat origin/main...feature/priority-routing
 router/priority.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git log --oneline origin/main..feature/priority-routing
2710851 Merge main into feature/priority-routing
fa6e912 Treat data loss as urgent
16d4788 Fix the name of the escalations queue
12ae95d Route high-priority tickets to escalation
44c1e7b Add priority scoring

# Ravi. One commit of his own, on a branch he created from main:
$ git log --oneline --graph origin/main origin/release/1.0 fix/empty-subject
* a1ae0eb Accept tickets without a subject
* 29be88c Describe how to run the tests
* 7e13c3d Raise the confidence threshold to 0.7
| * bc944fd Set version 1.0.0
|/  
* 9a383e5 Add classifier test
* f3e7ca9 Add routing config
* 53e7f57 Add keyword classifier
* fbbcc8d Add README

# The pull request was opened with base release/1.0. What it lists and shows:
$ git log --oneline origin/release/1.0..fix/empty-subject
a1ae0eb Accept tickets without a subject
29be88c Describe how to run the tests
7e13c3d Raise the confidence threshold to 0.7
$ git diff --stat origin/release/1.0...fix/empty-subject
 README.md           | 2 ++
 config/routing.yaml | 2 +-
 router/classify.py  | 2 +-
 3 files changed, 4 insertions(+), 2 deletions(-)

# Which base makes this branch a one-commit pull request?
$ for b in origin/main origin/release/1.0; do echo "$b: $(git rev-list --count $b..fix/empty-subject) commits"; done
origin/main: 1 commits
origin/release/1.0: 3 commits
$ git log --oneline -1 $(git merge-base origin/release/1.0 fix/empty-subject)
9a383e5 Add classifier test
$ git log --oneline -1 $(git merge-base origin/main fix/empty-subject)
29be88c Describe how to run the tests

# Fix A: the change belongs on main after all. Same branch, other base:
$ git log --oneline origin/main..fix/empty-subject
a1ae0eb Accept tickets without a subject
$ git diff --stat origin/main...fix/empty-subject
 router/classify.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

# Fix B: the change does belong on release/1.0. Move the one commit there:
$ git rebase --onto origin/release/1.0 origin/main fix/empty-subject
Rebasing (1/1)
Successfully rebased and updated refs/heads/fix/empty-subject.
$ git log --oneline origin/release/1.0..fix/empty-subject
5c46029 Accept tickets without a subject
$ git diff --stat origin/release/1.0...fix/empty-subject
 router/classify.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git log --oneline --graph origin/main origin/release/1.0 fix/empty-subject
* 5c46029 Accept tickets without a subject
* bc944fd Set version 1.0.0
| * 29be88c Describe how to run the tests
| * 7e13c3d Raise the confidence threshold to 0.7
|/  
* 9a383e5 Add classifier test
* f3e7ca9 Add routing config
* 53e7f57 Add keyword classifier
* fbbcc8d Add README

# In a copy where nothing was merged yet. Asha merges the UPPER pull request into main:
$ cd ../../asha-indirect/ticket-router
$ git fetch -q
$ git merge -q --no-ff -m "Merge pull request #2 from feature/sla-timers" origin/feature/sla-timers
$ git push -q origin main
# Is the head of the LOWER pull request now reachable from main?
$ git merge-base --is-ancestor origin/feature/priority-routing main
[exit status: 0]
$ git log --oneline main..origin/feature/priority-routing

# The upper pull request against main lists both layers:
$ git log --oneline origin/main..feature/sla-timers
60a7f43 Alert before the SLA of a high-priority ticket expires
32a2f99 Add SLA minutes per priority
9dcfb58 Fix the name of the escalations queue
02820ea Route high-priority tickets to escalation
3807b29 Add priority scoring
# Against the branch below it, it lists its own layer:
$ git log --oneline feature/priority-routing..feature/sla-timers
60a7f43 Alert before the SLA of a high-priority ticket expires
32a2f99 Add SLA minutes per priority
$ git diff --stat feature/priority-routing...feature/sla-timers
 config/routing.yaml | 1 +
 router/priority.py  | 5 +++++
 2 files changed, 6 insertions(+)

# Linear stack: the lower branch is an ancestor of the upper one.
$ git merge-base --is-ancestor feature/priority-routing feature/sla-timers
[exit status: 0]

# A review fix lands on the lower branch:
$ git switch -q feature/priority-routing
$ printf '\n\ndef test_outage_goes_to_escalations():\n    assert classify({"subject": "Outage in EU"}) == "escalations"\n' >> tests/test_classify.py
$ git commit -q -am "Test the escalations route"
$ git log --oneline --graph --decorate-refs=refs/heads feature/priority-routing feature/sla-timers -4
* c38d3bd Test the escalations route
| * 60a7f43 Alert before the SLA of a high-priority ticket expires
| * 32a2f99 Add SLA minutes per priority
|/  
* 9dcfb58 Fix the name of the escalations queue
$ git merge-base --is-ancestor feature/priority-routing feature/sla-timers
[exit status: 1]

# Restore the linear history: replay the upper layer on the new tip of the lower one.
$ git rebase feature/priority-routing feature/sla-timers
Rebasing (1/2)
Rebasing (2/2)
Successfully rebased and updated refs/heads/feature/sla-timers.
$ git merge-base --is-ancestor feature/priority-routing feature/sla-timers
[exit status: 0]
$ git log --oneline feature/priority-routing..feature/sla-timers
670af6f Alert before the SLA of a high-priority ticket expires
bf00849 Add SLA minutes per priority
$ git push -q origin feature/priority-routing
$ git push -q --force-with-lease origin feature/sla-timers

# The lower pull request was squash-merged and its branch deleted. The upper one now targets main:
$ git fetch --prune
From ../../server/ticket-router
 - [deleted]         (none)     -> origin/feature/priority-routing
   9a383e5..77fc160  main       -> origin/main
$ git log --oneline origin/main..feature/sla-timers
670af6f Alert before the SLA of a high-priority ticket expires
bf00849 Add SLA minutes per priority
c38d3bd Test the escalations route
9dcfb58 Fix the name of the escalations queue
02820ea Route high-priority tickets to escalation
3807b29 Add priority scoring
$ git merge-tree --write-tree --name-only origin/main feature/sla-timers
a9730e29529badbf0b65cf38bfcd5b884dde3c7b
router/priority.py

Auto-merging router/priority.py
CONFLICT (add/add): Merge conflict in router/priority.py
[exit status: 1]

# Only the upper layer is new. Transplant it: everything after the old lower branch, onto main.
$ git rebase --onto origin/main feature/priority-routing feature/sla-timers
Rebasing (1/2)
Rebasing (2/2)
Successfully rebased and updated refs/heads/feature/sla-timers.
$ git log --oneline origin/main..feature/sla-timers
41f9055 Alert before the SLA of a high-priority ticket expires
a791005 Add SLA minutes per priority
$ git merge-tree --write-tree origin/main feature/sla-timers
a59c7634e45ad06be44cb2ccab2a81fa40028b16
[exit status: 0]

   upstream: example-org/ticket-router  (Asha maintains)           your fork: you/ticket-router
  +--------------------------------------+   (1) fork             +------------------------------+
  | refs/heads/main                      | ---------------------> | refs/heads/main              |
  | refs/pull/7/head   <-----------------+-- (5) pull request --- | refs/heads/fix/empty-subject |
  +--------------------------------------+                        +------------------------------+
        |            ^                                                  ^            |
        | (3) fetch  | (7) merge by a maintainer                        | (4) push   | (2) clone
        | upstream   |                                                  | origin     |
        v            |                                                  |            v
  +-----------------------------------------------------------------------------------------+
  | your clone:  origin = your fork, upstream = the shared repository                        |
  |   main, fix/empty-subject         (6) review round trips: fetch, commit, push            |
  |   (8) after the merge: fetch upstream, fast-forward main, push main to origin            |
  +-----------------------------------------------------------------------------------------+

# Your clone of your fork. origin is the fork, upstream is the shared repository.
$ git remote -v
origin	../../forks/you/ticket-router.git (fetch)
origin	../../forks/you/ticket-router.git (push)
upstream	../../server/ticket-router.git (fetch)
upstream	../../server/ticket-router.git (push)
$ git fetch upstream
From ../../server/ticket-router
 * [new branch]      main       -> upstream/main
# Commits only upstream has (left) and only your fork has (right):
$ git rev-list --left-right --count upstream/main...origin/main
1	0

# Start from the current upstream, not from the stale main of the fork:
$ git switch -c fix/empty-subject --no-track upstream/main
Switched to a new branch 'fix/empty-subject'
$ git commit -q -am "Accept tickets without a subject"
$ git push -u origin fix/empty-subject
To ../../forks/you/ticket-router.git
 * [new branch]      fix/empty-subject -> fix/empty-subject
branch 'fix/empty-subject' set up to track 'origin/fix/empty-subject'.

# In the upstream repository. Opening pull request 7 from you:fix/empty-subject, as Git data:
$ git fetch -q ../../forks/you/ticket-router.git fix/empty-subject:refs/pull/7/head
$ git for-each-ref --format="%(objectname:short) %(refname)"
01822ed refs/heads/main
0c6455c refs/pull/7/head
# The commit is now stored in upstream, on no branch of upstream:
$ git cat-file -t refs/pull/7/head
commit
$ git branch --contains refs/pull/7/head

# Asha, a maintainer of upstream, takes the pull request into her clone:
$ git fetch origin pull/7/head:pr-7
From ../../server/ticket-router
 * [new ref]         refs/pull/7/head -> pr-7
$ git switch -q pr-7
$ git log --oneline main..pr-7
0c6455c Accept tickets without a subject

# She adds a test and pushes it to YOUR branch in YOUR fork (you allowed maintainer edits):
$ printf '\n\ndef test_ticket_without_subject():\n    assert classify({}) == "general"\n' >> tests/test_classify.py
$ git commit -q -am "Test a ticket without a subject"
$ git push ../../forks/you/ticket-router.git pr-7:fix/empty-subject
To ../../forks/you/ticket-router.git
   0c6455c..ef0b9f0  pr-7 -> fix/empty-subject

# You. Your branch in the fork moved without you:
$ git pull
From ../../forks/you/ticket-router
   0c6455c..ef0b9f0  fix/empty-subject -> origin/fix/empty-subject
Updating 0c6455c..ef0b9f0
Fast-forward
 tests/test_classify.py | 4 ++++
 1 file changed, 4 insertions(+)
$ git log --format="%h %an: %s" upstream/main..fix/empty-subject
ef0b9f0 Asha Rao: Test a ticket without a subject
0c6455c Lab User: Accept tickets without a subject

# You. Bring your clone and your fork up to date with upstream:
$ git switch -q main
$ git fetch upstream
From ../../server/ticket-router
   01822ed..fafe8b6  main       -> upstream/main
$ git merge --ff-only upstream/main
Updating 9a383e5..fafe8b6
Fast-forward
 config/routing.yaml    | 2 +-
 router/classify.py     | 2 +-
 tests/test_classify.py | 4 ++++
 3 files changed, 6 insertions(+), 2 deletions(-)
$ git push origin main
To ../../forks/you/ticket-router.git
   9a383e5..fafe8b6  main -> main
$ git rev-list --left-right --count upstream/main...origin/main
0	0

$ git branch -d fix/empty-subject
Deleted branch fix/empty-subject (was ef0b9f0).
$ git push origin --delete fix/empty-subject
To ../../forks/you/ticket-router.git
 - [deleted]         fix/empty-subject
$ git branch -a
* main
  remotes/origin/HEAD -> origin/main
  remotes/origin/main
  remotes/upstream/HEAD -> upstream/main
  remotes/upstream/main

# Create
gh pr create --base main --title "Accept tickets without a subject" --body "Fixes #12"
gh pr create --draft --fill                 # title and body from the commits; opens as a draft
gh pr create --dry-run                      # print what would be created (may still push)

# Inspect
gh pr status
gh pr list --state open --base main
gh pr view 12 --json baseRefName,headRefOid,mergeable,mergeStateStatus,reviewDecision
gh pr diff 12 --name-only
gh pr checks 12 --required                  # exit status 8 while checks are pending

# Review and iterate
gh pr checkout 12                           # local branch for the head of pull request 12
gh pr review 12 --approve                   # or --request-changes / --comment, with --body
gh pr ready 12                              # draft -> ready;  --undo goes back
gh pr edit 12 --base main                   # change the base branch
gh pr update-branch 12                      # merge the base into the head; --rebase to rebase

# Finish
gh pr merge 12 --squash --delete-branch
gh pr merge 12 --merge --match-head-commit "$(git rev-parse HEAD)"
gh pr merge 12 --auto --rebase
gh pr close 12 --comment "Superseded by #15" --delete-branch
gh pr revert 12