remote: disallow some nonsensical option combinations

remote: disallow some nonsensical option combinations

It doesn't make sense to use "-m" on a mirror, since "-m"
sets up the HEAD symref in the remotes namespace, but with
mirror, we are by definition not using a remotes namespace.

Similarly, it does not make much sense to specify refspecs
with --mirror. For a mirror you plan to push to, those
refspecs will be ignored. For a mirror you are fetching
from, there is no point in mirroring, since the refspec
specifies everything you want to grab.

There is one case where "--mirror -t <X>" would be useful.
Because <X> is used as-is in the refspec, and because we
append it to to refs/, you could mirror a subset of the
hierarchy by doing:

  git remote add --mirror -t 'tags/*'

But using anything besides a single branch as an argument to
"-t" is not documented and only happens to work, so closing
it off is not a serious regression.

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