Teach send-pack a mirror mode

Existing "git push --all" is almost perfect for backing up to
another repository, except that "--all" only means "all
branches" in modern git, and it does not delete old branches and
tags that exist at the back-up repository that you have removed
from your local repository.

This teaches "git-send-pack" a new "--mirror" option.  The
difference from the "--all" option are that (1) it sends all
refs, not just branches, and (2) it deletes old refs you no
longer have on the local side from the remote side.

Original patch by Junio C Hamano.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

send-pack.h

@@ -5,6 +5,7 @@ struct send_pack_args {
 	const char *receivepack;
 	unsigned verbose:1,
 		send_all:1,
+		send_mirror:1,
 		force_update:1,
 		use_thin_pack:1,
 		dry_run:1;