commit-reach(repo_in_merge_bases_many): optionally expect missing commits

Currently this function treats unrelated commit histories the same way
as commit histories with missing commit objects.

Typically, missing commit objects constitute a corrupt repository,
though, and should be reported as such. The next commits will make it
so, but there is one exception: In `git fetch --update-shallow` we
_expect_ commit objects to be missing, and we do want to treat the
now-incomplete commit histories as unrelated.

To allow for that, let's introduce an additional parameter that is
passed to `repo_in_merge_bases_many()` to trigger this behavior, and use
it in the two callers in `shallow.c`.

This commit changes behavior slightly: unless called from the
`shallow.c` functions that set the `ignore_missing_commits` bit, any
non-existing tip commit that is passed to `repo_in_merge_bases_many()`
will now result in an error.

Note: When encountering missing commits while traversing the commit
history in search for merge bases, with this commit there won't be a
change in behavior just yet, their children will still be interpreted as
root commits. This bug will get fixed by follow-up commits.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/remote.c b/remote.c
index e07b316eac..b3dbdd9b59 100644
--- a/remote.c
+++ b/remote.c
@@ -2680,7 +2680,7 @@ static int is_reachable_in_reflog(const char *local, const struct ref *remote)
 		if (MERGE_BASES_BATCH_SIZE < size)
 			size = MERGE_BASES_BATCH_SIZE;
 
-		if ((ret = repo_in_merge_bases_many(the_repository, commit, size, chunk)))
+		if ((ret = repo_in_merge_bases_many(the_repository, commit, size, chunk, 0)))
 			break;
 	}