Fixes a test case I received where a corrupted repo was repaired, but the
git-annex branch was not. The root of the problem was that the
MissingObject returned by the repair code was not necessarily a complete
set of all objects that might have been deleted during the repair.
So, stop trying to return that at all, and instead make the index file
checking code explicitly verify that each object the index uses is present.
Oh, git, you made this so hard. Not determining if a branch pointed to some
corrupt object, that was easy, but dealing with corrupt branches using git
plumbing is a PITA.
