Construct a list of ref prefixes to be passed to 'get_refs_list()' from
the refspec to be used during the push. This list of ref prefixes will
be used to allow the server to filter the ref advertisement when
communicating using protocol v2.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>