Fix git-pack-objects for 64-bit platforms

The offset of an object in the pack is recorded as a 4-byte integer
in the index file.  When reading the offset from the mmap'ed index
in prepare_pack_revindex(), the address is dereferenced as a long*.
This works fine as long as the long type is four bytes wide.  On
NetBSD/sparc64, however, a long is 8 bytes wide and so dereferencing
the offset produces garbage.

[jc: taking suggestion by Linus to use uint32_t]

Signed-off-by: Dennis Stosberg <dennis@stosberg.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>