[PATCH v4 2/2] revision: implement `git log --merge` also for rebase/cherry-pick/revert

[Philippe Blain <levraiphilippeblain@gmail.com>]

From: Michael Lohmann <mi.al.lohmann@gmail.com>

'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             |  8 ++++----
 Documentation/rev-list-options.txt |  6 ++++--
 revision.c                         | 31 +++++++++++++++++++++++--------
 3 files changed, 31 insertions(+), 14 deletions(-)

diff --git a/Documentation/gitk.txt b/Documentation/gitk.txt
index c2213bb77b..80ff4e149a 100644
--- a/Documentation/gitk.txt
+++ b/Documentation/gitk.txt
@@ -63,10 +63,10 @@ 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::
 
diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 2bf239ff03..5b4672c346 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -341,8 +341,10 @@ 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
diff --git a/revision.c b/revision.c
index aa4c4dc778..36dc2f94f7 100644
--- a/revision.c
+++ b/revision.c
@@ -1961,11 +1961,31 @@ static void add_pending_commit_list(struct rev_info *revs,
 	}
 }
 
+static const char *lookup_other_head(struct object_id *oid)
+{
+	int i;
+	static const char *const other_head[] = {
+		"MERGE_HEAD", "CHERRY_PICK_HEAD", "REVERT_HEAD", "REBASE_HEAD"
+	};
+
+	for (i = 0; i < ARRAY_SIZE(other_head); i++)
+		if (!read_ref_full(other_head[i],
+				RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+				oid, NULL)) {
+			if (is_null_oid(oid))
+				die("%s is a symbolic ref???", other_head[i]);
+			return other_head[i];
+		}
+
+	die("--merge without MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD or REBASE_HEAD?");
+}
+
 static void prepare_show_merge(struct rev_info *revs)
 {
 	struct commit_list *bases;
 	struct commit *head, *other;
 	struct object_id oid;
+	const char *other_name;
 	const char **prune = NULL;
 	int i, prune_num = 1; /* counting terminating NULL */
 	struct index_state *istate = revs->repo->index;
@@ -1973,15 +1993,10 @@ static void prepare_show_merge(struct rev_info *revs)
 	if (repo_get_oid(the_repository, "HEAD", &oid))
 		die("--merge without HEAD?");
 	head = lookup_commit_or_die(&oid, "HEAD");
-	if (read_ref_full("MERGE_HEAD",
-			RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
-			&oid, NULL))
-		die("--merge without MERGE_HEAD?");
-	if (is_null_oid(&oid))
-		die("MERGE_HEAD is a symbolic ref???");
-	other = lookup_commit_or_die(&oid, "MERGE_HEAD");
+	other_name = lookup_other_head(&oid);
+	other = lookup_commit_or_die(&oid, other_name);
 	add_pending_object(revs, &head->object, "HEAD");
-	add_pending_object(revs, &other->object, "MERGE_HEAD");
+	add_pending_object(revs, &other->object, other_name);
 	bases = repo_get_merge_bases(the_repository, head, other);
 	add_rev_cmdline_list(revs, bases, REV_CMD_MERGE_BASE, UNINTERESTING | BOTTOM);
 	add_pending_commit_list(revs, bases, UNINTERESTING | BOTTOM);

-- 
2.39.1