sequencer (rebase -i): suggest --edit-todo upon unknown command

This is the same behavior as known from `git rebase -i`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -1314,8 +1314,12 @@ static int read_populate_todo(struct todo_list *todo_list,
 	close(fd);
 
 	res = parse_insn_buffer(todo_list->buf.buf, todo_list);
-	if (res)
+	if (res) {
+		if (is_rebase_i(opts))
+			return error(_("please fix this using "
+				       "'git rebase --edit-todo'."));
 		return error(_("unusable instruction sheet: '%s'"), todo_file);
+	}
 
 	if (!todo_list->nr &&
 	    (!is_rebase_i(opts) || !file_exists(rebase_path_done())))