Split back out update_hook handling in receive-pack

Split back out update_hook handling in receive-pack

Since we have decided to change the calling conventions for the
pre-receive and post-receive hooks to take the ref data on stdin
rather than on the command line we cannot use the same logic to
invoke the update hook anymore.

So we take a small step backwards towards what we used to have,
and create a specialized function for executing just the update
hook.

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