push: make non-fast-forward help message configurable

push: make non-fast-forward help message configurable

This message is designed to help new users understand what
has happened when refs fail to push. However, it does not
help experienced users at all, and significantly clutters
the output, frequently dwarfing the regular status table and
making it harder to see.

This patch introduces a general configuration mechanism for
optional messages, with this push message as the first
example.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>