In the verison negotiation phase between "receive-pack" and
"proc-receive", "proc-receive" can send an empty flush-pkt to end the
negotiation and use default version 0. Capabilities (such as
"push-options") are not supported in version 0.
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>