Teach "git remote" a mirror mode

Teach "git remote" a mirror mode

When using the "--mirror" option to "git remote add", the refs will not
be stored in the refs/remotes/ namespace, but in the same location as
on the remote side.

This option probably only makes sense in a bare repository.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>