Make git-*-pull say who wants them for missing objects.

Make git-*-pull say who wants them for missing objects.

This patch updates pull.c, the engine that decides which objects are
needed, given a commit to traverse from, to report which commit was
calling for the object that cannot be retrieved from the remote side.
This complements git-fsck-cache in that it checks the consistency of
the remote repository for reachability.

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