transport: convert get_refs_list to take a list of ref prefixes

transport: convert get_refs_list to take a list of ref prefixes

Convert the 'struct transport' virtual function 'get_refs_list()' to
optionally take an argv_array of ref prefixes.  When communicating with
a server using protocol v2 these ref prefixes can be sent when
requesting a listing of their refs allowing the server to filter the
refs it sends based on the sent prefixes.  This list will be ignored
when not using protocol v2.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>