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.
As stated in the documentation: `Note that this hook WILL remove blank lines`
Previously this hook would always add blank a blank line.
With this fix, if the file is empty, a newline will not be added, and multpline blanklines in a file will be removed.
Fixes#935
Luis Pérez