pack-check.c::verify_packfile(): don't run SHA-1 update on huge data

pack-check.c::verify_packfile(): don't run SHA-1 update on huge data

Running the SHA1_Update() on the whole packfile in a single call
revealed an overflow problem we had in the SHA-1 implementation
on POWER architecture some time ago, which was fixed with commit
b47f509b (June 19, 2006).  Other SHA-1 implementations may have
a similar problem.

The sliding mmap() series already makes chunked calls to
SHA1_Update(), so this patch itself will become moot when it
graduates to "master", but in the meantime, run the hash
function in smaller chunks to prevent possible future problems.

Signed-off-by: Junio C Hamano <junkio@cox.net>