This makes both git-fetch and git-push (fetch-pack and receive-pack)
safe against a possible race with aparallel git-repack -a -d that could
prune the new pack while it is not yet referenced, and remove the .keep
file after refs have been updated.
Signed-off-by: Nicolas Pitre <nico@cam.org> Signed-off-by: Junio C Hamano <junkio@cox.net>