clone: add tags refspec earlier to fetch refspec

clone: add tags refspec earlier to fetch refspec

In clone.c we call refspec_ref_prefixes() to copy the fetch refspecs
from the `remote->fetch` refspec into `ref_prefixes` of
`transport_ls_refs_options`. Afterwards we add the tags prefix
`refs/tags/` prefix as well. At a later point, in wanted_peer_refs() we
process refs using both `remote->fetch` and `TAG_REFSPEC`.

Simplify the code by appending `TAG_REFSPEC` to `remote->fetch` before
calling refspec_ref_prefixes().

To be able to do this, we set `option_tags` to 0 when --mirror is given.
This is because --mirror mirrors (hence the name) all the refs,
including tags and they do not need to be treated separately.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>