bug#43162: chgrp clears setgid even when group is not changed

[Karl Berry]

Is it necessary for chgrp to clear setgid on directories even when the
group is not actually changed? In my life at least, it is rather
annoying. --thanks, karl.

$ mkdir foo
$ chmod g+s foo
$ ls -ld foo
drwxrwsr-x 2 karl root 6 Sep  1 10:36 foo/
$ chgrp root foo
$ ls -ld foo
drwxrwxr-x 2 karl root 6 Sep  1 10:36 foo/