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, 3 insertions, 2 deletions

diff --git a/shallow.c b/shallow.c
index 7711798127..998072d04a 100644
--- a/shallow.c
+++ b/shallow.c
@@ -796,7 +796,7 @@ static void post_assign_shallow(struct shallow_info *info,
 		for (j = 0; j < bitmap_nr; j++)
 			if (bitmap[0][j] &&
 			    /* Step 7, reachability test at commit level */
-			    !repo_in_merge_bases_many(the_repository, c, ca.nr, ca.commits)) {
+			    !repo_in_merge_bases_many(the_repository, c, ca.nr, ca.commits, 1)) {
 				update_refstatus(ref_status, info->ref->nr, *bitmap);
 				dst++;
 				break;
@@ -827,7 +827,8 @@ int delayed_reachability_test(struct shallow_info *si, int c)
 		si->reachable[c] = repo_in_merge_bases_many(the_repository,
 							    commit,
 							    si->nr_commits,
-							    si->commits);
+							    si->commits,
+							    1);
 		si->need_reachability_test[c] = 0;
 	}
 	return si->reachable[c];