also adds progress when actually writing a pack

also adds progress when actually writing a pack

If that pack is big, it takes significant time to write and might
benefit from some more eye candies as well.  This is however disabled
when the pack is written to stdout since in that case the output is
usually piped into unpack_objects which already does its own progress
reporting.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>