revision: implement `git log --merge` also for rebase/cherry-pick/revert

revision: implement `git log --merge` also for rebase/cherry-pick/revert

'git log' learned in ae3e5e1ef2 (git log -p --merge [[--] paths...],
2006-07-03) to show commits touching conflicted files in the range
HEAD...MERGE_HEAD, an addition documented in d249b45547 (Document
rev-list's option --merge, 2006-08-04).

It can be useful to look at the commit history to understand what lead
to merge conflicts also for other mergy operations besides merges, like
cherry-pick, revert and rebase.

For rebases and cherry-picks, an interesting range to look at is
HEAD...{REBASE_HEAD,CHERRY_PICK_HEAD}, since even if all the commits
included in that range are not directly part of the 3-way merge,
conflicts encountered during these operations can indeed be caused by
changes introduced in preceding commits on both sides of the history.

For revert, as we are (most likely) reversing changes from a previous
commit, an appropriate range is REVERT_HEAD..HEAD, which is equivalent
to REVERT_HEAD...HEAD and to HEAD...REVERT_HEAD, if we keep HEAD and its
parents on the left side of the range.

As such, adjust the code in prepare_show_merge so it constructs the
range HEAD...$OTHER for OTHER={MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD
or REBASE_HEAD}. Note that we try these pseudorefs in order, so keep
REBASE_HEAD last since the three other operations can be performed
during a rebase. Note also that in the uncommon case where $OTHER and
HEAD do not share a common ancestor, this will show the complete
histories of both sides since their root commits, which is the same
behaviour as currently happens in that case for HEAD and MERGE_HEAD.

Adjust the documentation of this option accordingly.

Co-authored-by: Johannes Sixt <j6t@kdbg.org>
Co-authored-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com>
[jc: tweaked in j6t's precedence fix that tries REBASE_HEAD last]
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>