(xstat): New global.

(xstat): New global.
(main): Define it.
(change_file_group): Use it.  Before this change, when running chgrp
on a symlink without --dereference (-h) and when the requested group
is the same as the group for the *symlink*, chgrp would do nothing.
Now it changes the group of the file referenced through the symlink.
Reported by Martin Mitchell.