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, 44 insertions, 8 deletions

diff --git a/Documentation/git-replay.adoc b/Documentation/git-replay.adoc
index 8d696ce3ab..997097e420 100644
--- a/Documentation/git-replay.adoc
+++ b/Documentation/git-replay.adoc
@@ -9,7 +9,7 @@ git-replay - EXPERIMENTAL: Replay commits on a new base, works with bare repos t
 SYNOPSIS
 --------
 [verse]
-(EXPERIMENTAL!) 'git replay' ([--contained] --onto <newbase> | --advance <branch>) [--ref-action[=<mode>]] <revision-range>
+(EXPERIMENTAL!) 'git replay' ([--contained] --onto <newbase> | --advance <branch> | --revert <branch>) [--ref-action[=<mode>]] <revision-range>
 
 DESCRIPTION
 -----------
@@ -42,6 +42,25 @@ The history is replayed on top of the <branch> and <branch> is updated to
 point at the tip of the resulting history. This is different from `--onto`,
 which uses the target only as a starting point without updating it.
 
+--revert <branch>::
+	Starting point at which to create the reverted commits; must be a
+	branch name.
++
+When `--revert` is specified, the commits in the revision range are reverted
+(their changes are undone) and the reverted commits are created on top of
+<branch>. The <branch> is then updated to point at the new commits. This is
+the same as running `git revert <revision-range>` but does not update the
+working tree.
++
+The commit messages follow `git revert` conventions: they are prefixed with
+"Revert" and include "This reverts commit <hash>." When reverting a commit
+whose message starts with "Revert", the new message uses "Reapply" instead.
+Unlike cherry-pick which preserves the original author, revert commits use
+the current user as the author, matching the behavior of `git revert`.
++
+This option is mutually exclusive with `--onto` and `--advance`. It is also
+incompatible with `--contained` (which is a modifier for `--onto` only).
+
 --contained::
 	Update all branches that point at commits in
 	<revision-range>. Requires `--onto`.
@@ -60,10 +79,11 @@ The default mode can be configured via the `replay.refAction` configuration vari
 
 <revision-range>::
 	Range of commits to replay; see "Specifying Ranges" in
-	linkgit:git-rev-parse[1]. In `--advance <branch>` mode, the
-	range should have a single tip, so that it's clear to which tip the
-	advanced <branch> should point. Any commits in the range whose
-	changes are already present in the branch the commits are being
+	linkgit:git-rev-parse[1]. In `--advance <branch>` or
+	`--revert <branch>` mode, the range should have a single tip,
+	so that it's clear to which tip the advanced or reverted
+	<branch> should point. Any commits in the range whose changes
+	are already present in the branch the commits are being
 	replayed onto will be dropped.
 
 :git-replay: 1
@@ -84,9 +104,10 @@ When using `--ref-action=print`, the output is usable as input to
 	update refs/heads/branch3 ${NEW_branch3_HASH} ${OLD_branch3_HASH}
 
 where the number of refs updated depends on the arguments passed and
-the shape of the history being replayed.  When using `--advance`, the
-number of refs updated is always one, but for `--onto`, it can be one
-or more (rebasing multiple branches simultaneously is supported).
+the shape of the history being replayed.  When using `--advance` or
+`--revert`, the number of refs updated is always one, but for `--onto`,
+it can be one or more (rebasing multiple branches simultaneously is
+supported).
 
 There is no stderr output on conflicts; see the <<exit-status,EXIT
 STATUS>> section below.
@@ -152,6 +173,21 @@ all commits they have since `base`, playing them on top of
 `origin/main`. These three branches may have commits on top of `base`
 that they have in common, but that does not need to be the case.
 
+To revert commits on a branch:
+
+------------
+$ git replay --revert main topic~2..topic
+------------
+
+This reverts the last two commits from `topic`, creating revert commits on
+top of `main`, and updates `main` to point at the result. This is useful when
+commits from `topic` were previously merged or cherry-picked into `main` and
+need to be undone.
+
+NOTE: For reverting an entire merge request as a single commit (rather than
+commit-by-commit), consider using `git merge-tree --merge-base $TIP HEAD $BASE`
+which can avoid unnecessary merge conflicts.
+
 GIT
 ---
 Part of the linkgit:git[1] suite