Use uint32_t for pack-objects counters.

Use uint32_t for pack-objects counters.

As we technically try to support up to a maximum of 2**32-1 objects
in a single packfile we should act like it and use unsigned 32 bit
integers for all of our object counts and progress output.

This change does not modify everything in pack-objects that probably
needs to change to fully support the maximum of 2**32-1 objects.
I'm intentionally breaking the improvements into slightly smaller
commits to make them easier to follow.

No logic change should be occuring here, with the exception that
some comparsions will now work properly when the number of objects
exceeds 2**31-1.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>