combine-diff: abort if --ignore-matching-lines is given

The code for combined diffs doesn't currently support ignoring changes
that match a regex.  Abort and report that fact instead of running into
a segfault.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1498,6 +1498,10 @@ void diff_tree_combined(const struct object_id *oid,
 	int i, num_paths, needsep, show_log_first, num_parent = parents->nr;
 	int need_generic_pathscan;
 
+	if (opt->ignore_regex_nr)
+		die("combined diff and '%s' cannot be used together",
+		    "--ignore-matching-lines");
+
 	/* nothing to do, if no parents */
 	if (!num_parent)
 		return;