Make xdi_diff_outf interface for running xdiff_outf diffs

Make xdi_diff_outf interface for running xdiff_outf diffs

To prepare for the need to initialize and release resources for an
xdi_diff with the xdiff_outf output function, make a new function to
wrap this usage.

Old:

	ecb.outf = xdiff_outf;
	ecb.priv = &state;
	...
	xdi_diff(file_p, file_o, &xpp, &xecfg, &ecb);

New:

	xdi_diff_outf(file_p, file_o, &state.xm, &xpp, &xecfg, &ecb);

Signed-off-by: Brian Downing <bdowning@lavos.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>