transport: add got_remote_refs flag

transport: add got_remote_refs flag

transport_get_remote_refs() in tranport.c checks transport->remote_refs
to determine whether transport->get_refs_list() should be invoked.  The
logic is "if it is NULL, we haven't run ls-remote to find out yet".

However, transport->remote_refs could still be NULL while cloning from
an empty repository.  This causes get_refs_list() to be run unnecessarily.

Introduce a flag, transport->got_remote_refs, to more explicitly record
if we have run transport->get_refs_list() already.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>