fetch_objects() currently does not set exact_oid in struct ref when
invoking transport_fetch_refs(). If the server supports ref-in-want,
fetch_pack() uses this field to determine whether a wanted ref should be
requested as a "want-ref" line or a "want" line; without the setting of
exact_oid, the wrong line will be sent.
Set exact_oid, so that the correct line is sent.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>