git-repack: Properly abort in corrupt repository

git-repack: Properly abort in corrupt repository

In a corrupt repository, git-repack produces a pack that does not
contain needed objects without complaining, and the result of this
combined with -d flag can be very painful -- e.g. a lossage of one
tree object can lead to lossage of blobs reachable only through that
tree.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>