Use chown --from to discover whether the

Use chown --from to discover whether the
group changed, since chgrp now changes unconditionally.  This
complicates the sed script a bit.  Do not specify --dereference,
since it's the default (and we want to test this).  Adjust output
to match the fact that chgrp no longer optimizes the case of
changing a file's group to the same value as before.