Construct a list of ref prefixes to be passed to
'transport_get_remote_refs()' from the refspec to be used during the
fetch. 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>