fetch: treat --tags like refs/tags/*:refs/tags/* when pruning

fetch: treat --tags like refs/tags/*:refs/tags/* when pruning

If --tags is specified, add that refspec to the list given to
prune_refs so it knows to treat it as a filter on what refs to
should consider for prunning. This way

    git fetch --prune --tags origin

only prunes tags and doesn't delete the branch refs.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>