New capability "report-status-v2" for git-push

New capability "report-status-v2" for git-push

The new introduced "proc-receive" hook may handle a command for a
pseudo-reference with a zero-old as its old-oid, while the hook may
create or update a reference with different name, different new-oid,
and different old-oid (the reference may exist already with a non-zero
old-oid).  Current "report-status" protocol cannot report the status for
such reference rewrite.

Add new capability "report-status-v2" and new report protocol which is
not backward compatible for report of git-push.

If a user pushes to a pseudo-reference "refs/for/master/topic", and
"receive-pack" creates two new references "refs/changes/23/123/1" and
"refs/changes/24/124/1", for client without the knowledge of
"report-status-v2", "receive-pack" will only send "ok/ng" directives in
the report, such as:

    ok ref/for/master/topic

But for client which has the knowledge of "report-status-v2",
"receive-pack" will use "option" directives to report more attributes
for the reference given by the above "ok/ng" directive.

    ok refs/for/master/topic
    option refname refs/changes/23/123/1
    option new-oid <new-oid>
    ok refs/for/master/topic
    option refname refs/changes/24/124/1
    option new-oid <new-oid>

The client will report two new created references to the end user.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>