use size_t to store pack .idx byte offsets

use size_t to store pack .idx byte offsets

We sometimes store the offset into a pack .idx file as an "unsigned
long", but the mmap'd size of a pack .idx file can exceed 4GB. This is
sufficient on LP64 systems like Linux, but will be too small on LLP64
systems like Windows, where "unsigned long" is still only 32 bits. Let's
use size_t, which is a better type for an offset into a memory buffer.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>