replay: add --revert mode to reverse commit changes

Add a `--revert <branch>` mode to git replay that undoes the changes
introduced by the specified commits. Like --onto and --advance, --revert
is a standalone mode: it takes a branch argument and updates that branch
with the newly created revert commits.

At GitLab, we need this in Gitaly for reverting commits directly on bare
repositories without requiring a working tree checkout.

The approach is the same as sequencer.c's do_pick_commit() -- cherry-pick
and revert are just the same three-way merge with swapped arguments:

  - Cherry-pick: merge(ancestor=parent, ours=current, theirs=commit)
  - Revert: merge(ancestor=commit, ours=current, theirs=parent)

We swap the base and pickme trees passed to merge_incore_nonrecursive()
to reverse the diff direction.

Reverts are processed newest-first (matching git revert behavior) to
reduce conflicts by peeling off changes from the top. Each revert
builds on the result of the previous one via the last_commit fallback
in the main replay loop, rather than relying on the parent-mapping
used for cherry-pick.

Revert commit messages follow the usual git revert conventions: prefixed
with "Revert" (or "Reapply" when reverting a revert), and including
"This reverts commit <hash>.". The author is set to the current user
rather than preserving the original author, matching git revert behavior.

Helped-by: Christian Couder <christian.couder@gmail.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Elijah Newren <newren@gmail.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 104 insertions, 7 deletions

diff --git a/t/t3650-replay-basics.sh b/t/t3650-replay-basics.sh
index a03f8f9293..217f6fb292 100755
--- a/t/t3650-replay-basics.sh
+++ b/t/t3650-replay-basics.sh
@@ -74,8 +74,8 @@ test_expect_success '--onto with invalid commit-ish' '
 	test_cmp expect actual
 '
 
-test_expect_success 'option --onto or --advance is mandatory' '
-	echo "error: option --onto or --advance is mandatory" >expect &&
+test_expect_success 'exactly one of --onto, --advance, or --revert is required' '
+	echo "error: exactly one of --onto, --advance, or --revert is required" >expect &&
 	test_might_fail git replay -h >>expect &&
 	test_must_fail git replay topic1..topic2 2>actual &&
 	test_cmp expect actual
@@ -87,16 +87,14 @@ test_expect_success 'no base or negative ref gives no-replaying down to root err
 	test_cmp expect actual
 '
 
-test_expect_success 'options --advance and --contained cannot be used together' '
-	printf "fatal: options ${SQ}--advance${SQ} " >expect &&
-	printf "and ${SQ}--contained${SQ} cannot be used together\n" >>expect &&
+test_expect_success '--advance and --contained cannot be used together' '
 	test_must_fail git replay --advance=main --contained \
 		topic1..topic2 2>actual &&
-	test_cmp expect actual
+	test_grep "cannot be used together" actual
 '
 
 test_expect_success 'cannot advance target ... ordering would be ill-defined' '
-	echo "fatal: cannot advance target with multiple sources because ordering would be ill-defined" >expect &&
+	echo "fatal: ${SQ}--advance${SQ} cannot be used with multiple revision ranges because the ordering would be ill-defined" >expect &&
 	test_must_fail git replay --advance=main main topic1 topic2 2>actual &&
 	test_cmp expect actual
 '
@@ -398,4 +396,103 @@ test_expect_success 'invalid replay.refAction value' '
 	test_grep "invalid.*replay.refAction.*value" error
 '
 
+test_expect_success 'argument to --revert must be a reference' '
+	echo "fatal: argument to --revert must be a reference" >expect &&
+	oid=$(git rev-parse main) &&
+	test_must_fail git replay --revert=$oid topic1..topic2 2>actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'cannot revert with multiple sources' '
+	echo "fatal: ${SQ}--revert${SQ} cannot be used with multiple revision ranges because the ordering would be ill-defined" >expect &&
+	test_must_fail git replay --revert main main topic1 topic2 2>actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'using replay --revert to revert commits' '
+	# Reuse existing topic4 branch (has commits I and J on top of main)
+	START=$(git rev-parse topic4) &&
+	test_when_finished "git branch -f topic4 $START" &&
+
+	# Revert commits I and J
+	git replay --revert topic4 topic4~2..topic4 &&
+
+	# Verify the revert commits were created (newest-first ordering
+	# means J is reverted first, then I on top)
+	git log --format=%s -4 topic4 >actual &&
+	cat >expect <<-\EOF &&
+	Revert "I"
+	Revert "J"
+	J
+	I
+	EOF
+	test_cmp expect actual &&
+
+	# Verify commit message format includes hash (tip is Revert "I")
+	test_commit_message topic4 <<-EOF &&
+	Revert "I"
+
+	This reverts commit $(git rev-parse I).
+	EOF
+
+	# Verify reflog message
+	git reflog topic4 -1 --format=%gs >reflog-msg &&
+	echo "replay --revert topic4" >expect-reflog &&
+	test_cmp expect-reflog reflog-msg
+'
+
+test_expect_success 'using replay --revert in bare repo' '
+	# Reuse existing topic4 in bare repo
+	START=$(git -C bare rev-parse topic4) &&
+	test_when_finished "git -C bare update-ref refs/heads/topic4 $START" &&
+
+	# Revert commit J in bare repo
+	git -C bare replay --revert topic4 topic4~1..topic4 &&
+
+	# Verify revert was created
+	git -C bare log -1 --format=%s topic4 >actual &&
+	echo "Revert \"J\"" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'revert of revert uses Reapply' '
+	# Use topic4 and first revert J, then revert the revert
+	START=$(git rev-parse topic4) &&
+	test_when_finished "git branch -f topic4 $START" &&
+
+	# First revert J
+	git replay --revert topic4 topic4~1..topic4 &&
+	REVERT_J=$(git rev-parse topic4) &&
+
+	# Now revert the revert - should become Reapply
+	git replay --revert topic4 topic4~1..topic4 &&
+
+	# Verify Reapply prefix and message format
+	test_commit_message topic4 <<-EOF
+	Reapply "J"
+
+	This reverts commit $REVERT_J.
+	EOF
+'
+
+test_expect_success 'git replay --revert with conflict' '
+	# conflict branch has C.conflict which conflicts with topic1s C
+	test_expect_code 1 git replay --revert conflict B..topic1
+'
+
+test_expect_success 'git replay --revert incompatible with --contained' '
+	test_must_fail git replay --revert topic4 --contained topic4~1..topic4 2>error &&
+	test_grep "cannot be used together" error
+'
+
+test_expect_success 'git replay --revert incompatible with --onto' '
+	test_must_fail git replay --revert topic4 --onto main topic4~1..topic4 2>error &&
+	test_grep "cannot be used together" error
+'
+
+test_expect_success 'git replay --revert incompatible with --advance' '
+	test_must_fail git replay --revert topic4 --advance main topic4~1..topic4 2>error &&
+	test_grep "cannot be used together" error
+'
+
 test_done