Don't run post-update hook unless a ref changed

Don't run post-update hook unless a ref changed

There is little point in executing the post-update hook if all refs
had an error and were unable to be updated.  In this case nothing
new is reachable within the repository, and there is no state change
for the post-update hook to be interested in.

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