diff-delta: bound hash list length to avoid O(m*n) behavior

diff-delta: bound hash list length to avoid O(m*n) behavior

The diff-delta code can exhibit O(m*n) behavior with some patological
data set where most hash entries end up in the same hash bucket.

The latest code rework reduced the block size making it particularly
vulnerable to this issue, but the issue was always there and can be
triggered regardless of the block size.

This patch does two things:

1) the hashing has been reworked to offer a better distribution to
   atenuate the problem a bit, and

2) a limit is imposed to the number of entries that can exist in the
   same hash bucket.

Because of the above the code is a bit more expensive on average, but
the problematic samples used to diagnoze the issue are now orders of
magnitude less expensive to process with only a slight loss in
compression.

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