replace adler32 with Rabin's polynomial in diff-delta

replace adler32 with Rabin's polynomial in diff-delta

This brings another small repacking speedup for sensibly the same pack
size.  On the Linux kernel repo, git-repack -a -f is 3.7% faster for a
0.4% larger pack.

Credits to Geert Bosch who brought the Rabin's polynomial idea to my
attention.

This also eliminate the issue of adler32() reading past the data buffer,
as noticed by Johannes Schindelin.

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