Some callers of xdi_diff_outf() do not look at the generated hunk header
lines at all. By plugging in a no-op hunk callback, this tells xdiff not
to even bother formatting them.
This patch introduces a stock no-op callback and uses it with a few
callers whose line callbacks explicitly ignore hunk headers (because
they look only for +/- lines).
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>