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>

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