Refactor run_update_hook to be more useful

Refactor run_update_hook to be more useful

This is a simple refactoring of run_update_hook to allow the function
to be passed the name of the hook it runs and also to build the
argument list from a list of struct commands, rather than just one
struct command.

The refactoring is to support new pre-receive and post-receive
hooks that will be given the entire list of struct commands,
rather than just one struct command.  These new hooks will follow
in another patch.

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