builtin/unpack-objects.c: show throughput progress

builtin/unpack-objects.c: show throughput progress

'git unpack-objects' shows a progress line only counting the number of
unpacked objects, so if some of the received objects are unusually
large, then that progress might appear to be frozen while processing
such a larger object.  I just stared at a seemingly stuck progress
line for over half a minute, while 'git fetch' was busy receiving a
pack with only a couple of objects (i.e. fewer than
'fetch.unpackLimit'), with one of them being over 80MB.

Display throughput in 'git unpack-objects' progress line, so we show
that something is going on even when receiving and processing a large
object.

Counting the consumed bytes is far away from the place that
counts objects and displays progress, and to pass around the 'struct
progress' instance we would have to modify the signature of five
functions and 14 of their callsites: this is just too much churn, so
let's rather make it file-scope static.

'git index-pack', i.e. the non-unpacking cousin of 'git
unpack-objects' already includes throughput in its progress line, and
it uses a file-scope static 'struct progress' instance as well.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>