[PATCH v4 0/2] Implement `git log --merge` also for rebase/cherry-pick/revert

Changes in v4:
- Added a commit message for 2/2 detailing the use case and summarizing the discussion in the thread
- Adjusted the documentation of the option

Link to v3: https://lore.kernel.org/r/20240117081405.14012-1-mi.al.lohmann@gmail.com

Range-diff vs v3:

1:  37405be1a3 = 1:  37405be1a3 revision: ensure MERGE_HEAD is a ref in prepare_show_merge
2:  de080a628c ! 2:  6ac1608809 revision: implement `git log --merge` also for rebase/cherry_pick/revert
    @@ Metadata
     Author: Michael Lohmann <mi.al.lohmann@gmail.com>

      ## Commit message ##
    -    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, an interesting range to look at is HEAD...REBASE_HEAD,
    +    since the conflicts are usually caused by how the code changed
    +    differently on HEAD since REBASE_HEAD forked from it.
    +
    +    For cherry-picks and revert, it is less clear that
    +    HEAD...CHERRY_PICK_HEAD and HEAD...REVERT_HEAD are indeed interesting
    +    ranges, since these commands are about applying or unapplying a single
    +    (or a few, for cherry-pick) commit(s) on top of HEAD. However, conflicts
    +    encountered during these operations can indeed be caused by changes
    +    introduced in preceding commits on both sides of the history.
    +
    +    Adjust the code in prepare_show_merge so it constructs the range
    +    HEAD...$OTHER for each of 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: Philippe Blain <levraiphilippeblain@gmail.com>
         Co-authored-by: Johannes Sixt <j6t@kdbg.org>
    +    Signed-off-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>

    + ## Documentation/gitk.txt ##
    +@@ Documentation/gitk.txt: linkgit:git-rev-list[1] for a complete list.
    +
    + --merge::
    +
    +-	After an attempt to merge stops with conflicts, show the commits on
    +-	the history between two branches (i.e. the HEAD and the MERGE_HEAD)
    +-	that modify the conflicted files and do not exist on all the heads
    +-	being merged.
    ++	Show commits touching conflicted paths in the range `HEAD...$OTHER`,
    ++	where `$OTHER` is the first existing pseudoref in `MERGE_HEAD`,
    ++	`CHERRY_PICK_HEAD`, `REVERT_HEAD` or `REBASE_HEAD`. Only works
    ++	when the index has unmerged entries.
    +
    + --left-right::
    +
    +
    + ## Documentation/rev-list-options.txt ##
    +@@ Documentation/rev-list-options.txt: See also linkgit:git-reflog[1].
    + Under `--pretty=reference`, this information will not be shown at all.
    +
    + --merge::
    +-	After a failed merge, show refs that touch files having a
    +-	conflict and don't exist on all heads to merge.
    ++	Show commits touching conflicted paths in the range `HEAD...$OTHER`,
    ++	where `$OTHER` is the first existing pseudoref in `MERGE_HEAD`,
    ++	`CHERRY_PICK_HEAD`, `REVERT_HEAD` or `REBASE_HEAD`. Only works
    ++	when the index has unmerged entries.
    +
    + --boundary::
    + 	Output excluded boundary commits. Boundary commits are
    +
      ## revision.c ##
     @@ revision.c: static void add_pending_commit_list(struct rev_info *revs,
      	}

---
Michael Lohmann (2):
      revision: ensure MERGE_HEAD is a ref in prepare_show_merge
      revision: implement `git log --merge` also for rebase/cherry-pick/revert

 Documentation/gitk.txt             |  8 ++++----
 Documentation/rev-list-options.txt |  6 ++++--
 revision.c                         | 27 +++++++++++++++++++++++----
 3 files changed, 31 insertions(+), 10 deletions(-)
---
base-commit: 186b115d3062e6230ee296d1ddaa0c4b72a464b5
change-id: 20240210-ml-log-merge-with-cherry-pick-and-other-pseudo-heads-05bd8e8797db