upload-pack: bump keepalive default to 5 seconds

There is no reason not to turn on keepalives by default.
They take very little bandwidth, and significantly less than
the progress reporting they are replacing. And in the case
that progress reporting is on, we should never need to send
a keepalive anyway, as we will constantly be showing
progress and resetting the keepalive timer.

We do not necessarily know what the client's idea of a
reasonable timeout is, so let's keep this on the low side of
5 seconds. That is high enough that we will always prefer
our normal 1-second progress reports to sending a keepalive
packet, but low enough that no sane client should consider
the connection hung.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>