xdl_diff: identify call sites.

This inserts a new function xdi_diff() that currently does not
do anything other than calling the underlying xdl_diff() to the
callchain of current callers of xdl_diff() function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-tree.c

@@ -119,7 +119,7 @@ static void show_diff(struct merge_list *entry)
 	if (!dst.ptr)
 		size = 0;
 	dst.size = size;
-	xdl_diff(&src, &dst, &xpp, &xecfg, &ecb);
+	xdi_diff(&src, &dst, &xpp, &xecfg, &ecb);
 	free(src.ptr);
 	free(dst.ptr);
 }