[PATCH] merge-recursive: fix the diff3 common ancestor label for virtual commits

[Elijah Newren <newren@xxxxxxxxx>]

In commit 743474cbfa8b ("merge-recursive: provide a better label for
diff3 common ancestor", 2019-08-17), the label for the common ancestor
was changed from always being

         "merged common ancestors"

to instead be based on the number of merge bases:

    >=2: "merged common ancestors"
      1: <abbreviated commit hash>
      0: "<empty tree>"

Unfortunately, this did not take into account that when we have a single
merge base, that merge base could be fake or constructed.  In such
cases, this resulted in a label of "00000000".  Of course, the previous
label of "merged common ancestors" was also misleading for these cases.
Since we have an API that is explicitly about creating fake commits in
merge_recursive_generic(), we should provide a better label when using
that API.  So, when merge_recursive_generic() is called, set the label
to:

         "constructed merge base"

Note that users of merge_recursive_generic include the builtin commands
git-am (in combination with git apply --build-fake-ancestor),
git-merge-recursive, and git-stash.

Reported-by: Jeff King <peff@xxxxxxxx>
Signed-off-by: Elijah Newren <newren@xxxxxxxxx>
---
Applies to the top of en/merge-recursive-cleanup, which is in next.

 merge-recursive.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/merge-recursive.c b/merge-recursive.c
index b058741f00..2b7722e350 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -3550,6 +3550,8 @@ static int merge_recursive_internal(struct merge_options *opt,
 		merged_merge_bases = make_virtual_commit(opt->repo, tree,
 							 "ancestor");
 		ancestor_name = "empty tree";
+	} else if (opt->ancestor) {
+		ancestor_name = opt->ancestor;
 	} else if (merge_bases) {
 		ancestor_name = "merged common ancestors";
 	} else {
@@ -3689,7 +3691,8 @@ int merge_recursive(struct merge_options *opt,
 {
 	int clean;
 
-	assert(opt->ancestor == NULL);
+	assert(opt->ancestor == NULL ||
+	       !strcmp(opt->ancestor, "constructed merge base"));
 
 	if (merge_start(opt, repo_get_commit_tree(opt->repo, h1)))
 		return -1;
@@ -3741,6 +3744,7 @@ int merge_recursive_generic(struct merge_options *opt,
 					   oid_to_hex(merge_bases[i]));
 			commit_list_insert(base, &ca);
 		}
+		opt->ancestor = "constructed merge base";
 	}
 
 	repo_hold_locked_index(opt->repo, &lock, LOCK_DIE_ON_ERROR);
-- 
2.23.0.25.ge3b5015985