Provides a constructor for a Difference that does nothing on a call to 'clean' and saves the hash/last modified time of the files as they were prior to running the function.
Provides a constructor for a Difference that removes the files from the previous run on a call to 'clean' and saves the hash/last modified time of the files as they are after running the function.
Provides a constructor for a Difference that removes the files from the previous run on a call to 'clean' and saves the hash/last modified time of the files as they are after running the function. This means that this information must be evaluated twice: before and after running the function.