Move post-update hook to after all other activity

Move post-update hook to after all other activity

As the post-update hook is meant to run after we have completed the
receipt of the pushed changes, and it might actually try to kick off
a `repack -a -d`, we should delay on invoking it until after we have
removed the *.keep file on the uploaded pack (if we kept the pack).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>