reflog expire: prune commits that are not incomplete

reflog expire: prune commits that are not incomplete

Older fsck-objects and prune did not protect commits in reflog
entries, and it is quite possible that a commit still exists in
the repository (because it was in a pack, or something) while
some of its trees and blobs are long gone.  Make sure the commit
and its associated tree is complete and expire incomplete ones.

Signed-off-by: Junio C Hamano <junkio@cox.net>