fetch-pack: use ref adv. to prune "have" sent

fetch-pack: use ref adv. to prune "have" sent

In negotiation using protocol v2, fetch-pack sometimes does not make
full use of the information obtained in the ref advertisement:
specifically, that if the server advertises a commit that the client
also has, the client never needs to inform the server that it has the
commit's parents, since it can just tell the server that it has the
advertised commit and it knows that the server can and will infer the
rest.

This is because, in do_fetch_pack_v2(), rev_list_insert_ref_oid() is
invoked before mark_complete_and_common_ref(). This means that if we
have a commit that is both our ref and their ref, it would be enqueued
by rev_list_insert_ref_oid() as SEEN, and since it is thus already SEEN,
mark_complete_and_common_ref() would not enqueue it.

If mark_complete_and_common_ref() were invoked first, as it is in
do_fetch_pack() for protocol v0, then mark_complete_and_common_ref()
would enqueue it with COMMON_REF | SEEN. The addition of COMMON_REF
ensures that its parents are not sent as "have" lines.

Change the order in do_fetch_pack_v2() to be consistent with
do_fetch_pack(), and to avoid sending unnecessary "have" lines.

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