pseudo-merge.c: ensure pseudo-merge groups are closed

When generating pseudo-merge bitmaps, it's possible that concurrent
reference updates may reveal some pseudo-merge candidates which reach
objects that are not contained in the bitmap's pack or pseudo-pack
order (in the case of MIDX bitmaps).

The latter case is relatively easy to demonstrate: if we generate a MIDX
bitmap with only half of the repository packed, then the unpacked
contents are not part of the MIDX's object order.

If we happen to select one or more commit(s) from the unpacked portion
of the repository for inclusion in a pseudo-merge, we'll get the
following message when trying to generate its bitmap:

    $ git multi-pack-index write --bitmap
    [...]
    Selecting pseudo-merge commits: 100% (1/1), done.
    warning: Failed to write bitmap index. Packfile doesn't have full closure (object ... is missing)
    Building bitmaps:  50% (1/2), done.
    error: could not write multi-pack bitmap

, and the attempted bitmap write will fail, leaving the repository
without a current bitmap.

Rectify this by ensuring that the commits which are pseudo-merge
candidates can only be so if they appear somewhere in the packing order.

This is sufficient, since we know that the original packing order is
closed under reachability, so if a commit appears in that list as a
potential pseudo-merge candidate, we know that everything reachable from
it also appears in the list (and thus the candidate is a good one).

Noticed-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pseudo-merge.c

@@ -217,6 +217,8 @@ static int find_pseudo_merge_group_for_ref(const char *refname,
 	c = lookup_commit(the_repository, oid);
 	if (!c)
 		return 0;
+	if (!packlist_find(writer->to_pack, oid))
+		return 0;
 
 	has_bitmap = bitmap_writer_has_bitmapped_object_id(writer, oid);