"git-push $URL" without refspecs pushes only matching branches

"git-push $URL" without refspecs pushes only matching branches

When "git push" is run without any refspec (neither on the
command line nor in the config), we used to push "matching refs"
in the sense that anything under refs/ hierarchy that exist on
both ends were updated.  This used to be a sane default for
publishing your repository to another back when we did not have
refs/remotes/ hierarchy, but it does not make much sense these
days.

This changes the semantics to push only "matching branches".

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