progress: show overall rate in last update

progress: show overall rate in last update

The values in struct throughput are only updated every 0.5 seconds.  If
we're all done before that time span then the final update will show a
rate of 0 bytes/s, which is misleading if some bytes had been handled.
Remember the start time and show the total throughput instead.

And avoid division by zero by enforcing a minimum time span value of 1
(unit: 1/1024th of a second).  That makes the resulting rate an
underestimation, but it's closer to the actual value than the currently
shown 0 bytes/s.

Reported-by: 積丹尼 Dan Jacobson <jidanni@jidanni.org>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>