hook/update: example of how to prevent branch creation

hook/update: example of how to prevent branch creation

Since git doesn't provide a receive.denyBranchCreation or similar, here is
an example of how to be sure users cannot create branches remotely by
pushing a new reference.

This setup has been proven useful to prevent creation of spurious branches
because of users having their remote.origin.push set to HEAD, when they
use `git push` while being on a local topic branch of theirs instead of
the proper one.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>