Refuse updating the current branch in a non-bare repository via push

Refuse updating the current branch in a non-bare repository via push

This makes git-push refuse pushing into a non-bare repository to update
the current branch by default.  To help people who are used to be able to
do this (and later "reset --hard" it in some other way), an error message
is issued when this refusal is triggered, instructing how to resurrect the
old behaviour.

Hosting sites that do not give the users direct access to customize their
repositories (e.g. repo.or.cz, gitorious, github etc.) may further want to
explicitly set the configuration variable to "refuse" for their customers'
repositories.

Signed-off-by: Junio C Hamano <gitster@pobox.com>