Disallow --unique alongside --ignore-cases in file-contents-sorter

Closes #794: using these flags at the same time results in undefined
behavior - the final ordering is not guaranteed as --unique breaks
sorting due to passing the contents through a set.

NOTE: I added a newline before and after the mutex group to be
consistent with other usages of mutex groups (e.g.,
check_builtin_literals.py) in this repo.