sha1_file.c: learn about index version 2

sha1_file.c: learn about index version 2

With this patch, packs larger than 4GB are usable, even on a 32-bit machine
(at least on Linux).  If off_t is not large enough to deal with a large
pack then die() is called instead of attempting to use the pack and
producing garbage.

This was tested with a 8GB pack specially created for the occasion on
a 32-bit machine.

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