push: always load default config

push: always load default config

This is needed because we want to use the
advice.pushnonfastforward variable.

Previously, we would load the config on demand only when we
needed to look at push.default.  Which meant that "git push"
would load it, but "git push remote" would not, leading to
differing behavior.

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