git-repack: Be careful when updating the same pack as an existing one.

git-repack: Be careful when updating the same pack as an existing one.

After a clone, packfiles are read-only by default and "mv" to
replace the pack with a new one goes interactive, asking if the
user wants to replace it.  If one is successfully moved and the
other is not, the pack and its idx would become out-of-sync and
corrupts the repository.

Recovering is straightforward -- it is just the matter of
finding the remaining .tmp-pack-* and make sure they are both
moved -- but we should be extra careful not to do something so
alarming to the users.

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