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>