Git.pm: Add remote_refs() git-ls-remote frontend

Git.pm: Add remote_refs() git-ls-remote frontend

This patch also converts the good ole' git-remote.perl to use it.
It is otherwise used in the repo.or.cz machinery and I guess other
scripts might find it useful too.

Unfortunately,

	git-ls-remote --heads .

is subtly different from

	git-ls-remote . refs/heads/

(since the second matches anywhere in the string, not just at the
beginning) so we have to provide interface for both.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>