Teach clone to send a list of ref-prefixes, when using protocol v2, to
allow the server to filter out irrelevant references from the
ref-advertisement. This reduces wasted time and bandwidth when cloning
repositories with a larger number of references.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>