combine-diff: remove misguided --show-empty hack.

Now --always flag is available in diff-tree, there is no reason
to have that hack in the diffcore side.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff-files.c

@@ -160,7 +160,7 @@ int main(int argc, const char **argv)
 			if (combine_merges && num_compare_stages == 2) {
 				show_combined_diff(&combine.p, 2,
 						   dense_combined_merges,
-						   NULL, 0);
+						   NULL);
 				free(combine.p.path);
 				continue;
 			}