transport: do not list refs if possible

transport: do not list refs if possible

When all refs to be fetched are exact OIDs, it is possible to perform a
fetch without requiring the remote to list refs if protocol v2 is used.
Teach Git to do this.

This currently has an effect only for lazy fetches done from partial
clones. The change necessary to likewise optimize "git fetch <remote>
<sha-1>" will be done in a subsequent patch.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>