hook: add sample template for push-to-checkout

hook: add sample template for push-to-checkout

The template is a more-or-less exact translation to shell of the C
code for the default behaviour for git's push-to-checkout hook defined
in the push_to_deploy() function in builtin/receive-pack.c, to serve
as a convenient starting point for modification.

It also contains relevant text extracted from the git-config(1) and
githooks(5) man pages.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>