Jeff King [off-list ref] writes:
If the new client can handle the old-style server's response, then the
server can start blasting out refs (optionally after a timeout) and stop
when the client interrupts with "hey, wait, I can speak the new
protocol". The server just has to include "you can interrupt me" in its
capability advertisement (obviously it would have to send out at least
the first ref with the capabilities before the timeout).
Yeah, I would prefer people to come up with a way to share the port
and autodetect. It is *not* a requirement for the updated server to
run on a separate port at all.