upload-pack: prefer flushing data over sending keepalive

upload-pack: prefer flushing data over sending keepalive

When using the sideband in git-upload-pack(1) we know to send out
keepalive packets in case generating the pack takes too long. These
keepalives take the form of a simple empty pktline.

In the preceding commit we have adapted git-upload-pack(1) to buffer
data more aggressively before sending it to the client. This creates an
obvious optimization opportunity: when we hit the keepalive timeout
while we still hold on to some buffered data, then it makes more sense
to flush out the data instead of sending the empty keepalive packet.

This is overall not going to be a significant win. Most keepalives will
come before the pack data starts, and once pack-objects starts producing
data, it tends to do so pretty consistently. And of course we can't send
data before we see the PACK header, because the whole point is to buffer
the early bit waiting for packfile URIs. But the optimization is easy
enough to realize.

Do so and flush out data instead of sending an empty pktline.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>