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 <[email protected]>

Author: dscho

sequencer.c

@@ -1343,8 +1343,12 @@ 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())))