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

hildeth/chapel,chizarlicious/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,chizarlicious/chapel,hildeth/chapel,CoryMcCartan/chapel,chizarlicious/chapel,CoryMcCartan/chapel,hildeth/chapel,CoryMcCartan/chapel,hildeth/chapel,hildeth/chapel,chizarlicious/chapel,CoryMcCartan/chapel,hildeth/chapel,chizarlicious/chapel,chizarlicious/chapel,chizarlicious/chapel,hildeth/chapel,CoryMcCartan/chapel