xdiff-interface: provide a separate consume callback for hunks

xdiff-interface: provide a separate consume callback for hunks

The previous commit taught xdiff to optionally provide the hunk header
data to a specialized callback. But most users of xdiff actually use our
more convenient xdi_diff_outf() helper, which ensures that our callbacks
are always fed whole lines.

Let's plumb the special hunk-callback through this interface, too. It
will follow the same rule as xdiff when the hunk callback is NULL (i.e.,
continue to pass a stringified hunk header to the line callback). Since
we add NULL to each caller, there should be no behavior change yet.

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