combine-diff: use an xdiff hunk callback

combine-diff: use an xdiff hunk callback

A combined diff has to line up the hunks for all of the individual
pairwise diffs, and thus needs to know their line numbers and sizes. We
get that now by parsing the hunk header line that xdiff generates.
However, now that xdiff supports a hunk callback, we can just use the
values directly.

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