remote: deprecate --mirror

remote: deprecate --mirror

The configuration created by plain --mirror is dangerous and
useless, and we now have --mirror=fetch and --mirror=push to
replace it. Let's warn the user.

One alternative to this is to try to guess which type the
user wants. In a non-bare repository, a fetch mirror doesn't
make much sense, since it would overwrite local commits. But
in a bare repository, you might use either type, or even
both (e.g., if you are acting as an intermediate drop-point
across two disconnected networks).

So rather than try for complex heuristics, let's keep it
simple. The user knows what they're trying to do, so let
them tell us.

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