Use off_t in pack-objects/fast-import when we mean an offset

Always use an off_t value in pack-objects anytime we are dealing
with an offset to some data within a packfile.

Also fixed a minor uintmax_t that was incorrectly defined before.

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