diffcore-delta.c: update the comment on the algorithm.

diffcore-delta.c: update the comment on the algorithm.

The comment at the top of the file described an old algorithm
that was neutral to text/binary differences (it hashed sliding
window of N-byte sequences and counted overlaps), but long time
ago we switched to a new heuristics that are more suitable for
line oriented (read: text) files that are much faster.

Signed-off-by: Junio C Hamano <gitster@pobox.com>