Add sample pre-push hook script

Add sample pre-push hook script

Create a sample of a script for a pre-push hook.  The main purpose is to
illustrate how a script may parse the information which is supplied to
such a hook.  The script may also be useful to some people as-is for
avoiding to push commits which are marked as a work in progress.

Signed-off-by: Aaron Schrab <aaron@schrab.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>