Fix behavior of --todo with an optional argument

The previous behavior of --todo was to always only list uncompleted
items, unless zero was specified as additional argument. Restore and
document this behavior.

Also, fix two tests that failed because the --todo output is now sorted.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/args.c

@@ -534,6 +534,8 @@ int parse_args(int argc, char **argv)
 				filter.priority = atoi(optarg);
 				if (filter.priority == 0)
 					filter.completed = 1;
+				else
+					filter.uncompleted = 1;
 				EXIT_IF(filter.priority > 9,
 					_("invalid priority: %s"), optarg);
 			} else {