diff: use hunk callback for word-diff

diff: use hunk callback for word-diff

Our word-diff does not look at the -/+ lines generated by xdiff at all
(because they are not real lines to show the user, but just the
tokenized words split into lines). Instead we use the line numbers from
the hunk headers to index our own data structure.

As a result, our xdi_diff_outf() callback throws away all lines except
hunk headers. We can instead use a hunk callback, which has two
benefits:

  1. We don't have to re-parse the generated hunk header line, but can
     use the passed parameters directly.

  2. By setting our line callback to NULL, we can tell xdiff-interface
     that it does not even need to bother generating the other lines,
     saving a small amount of work.

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