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.

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 [off-list ref]
Helped-by: Patrick Steinhardt [off-list ref]
Helped-by: Elijah Newren [off-list ref]
Helped-by: Phillip Wood [off-list ref]
Helped-by: Johannes Schindelin [off-list ref]
Helped-by: Junio C Hamano [off-list ref]
Signed-off-by: Siddharth Asthana <redacted>
---
 Documentation/git-replay.adoc |  37 +++++++-
 builtin/replay.c              |  25 ++++--
 replay.c                      | 162 ++++++++++++++++++++++++----------
 replay.h                      |  11 ++-
 t/t3650-replay-basics.sh      | 107 ++++++++++++++++++++--
 5 files changed, 277 insertions(+), 65 deletions(-)

diff --git a/Documentation/git-replay.adoc b/Documentation/git-replay.adoc
index 8d696ce3ab..ffdf790278 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>...