Update chown/nothing.prediff to check all user groups.

The group assigned to a particular file will not necessarily be the group
reported by `$(id -gn)`. Update nothing.prediff to check that the group
assigned to the file is one of the groups reported by `$(groups)`.

bash-ify the prediff along the way.
Merge pull request #834 from thomasvandoren/robustify-chown-nothing-test

Update chown/nothing.prediff to check all user groups.

The group assigned to a particular file will not necessarily be the group
reported by $(id -gn). Update nothing.prediff to check that the group
assigned to the file is one of the groups reported by $(groups).

bash-ify the prediff along the way.