diff: use large integers for diffstat calculations

diff: use large integers for diffstat calculations

The diffstat "added" and "changed" fields generally store
line counts; however, for binary files, they store file
sizes. Since we store and print these values as ints, a
diffstat on a file larger than 2G can show a negative size.
Instead, let's use uintmax_t, which should be at least 64
bits on modern platforms.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>