Look into hard links

[Dr-Emann]

See RJVB/afsctool#61 (comment), it might not be safe to compress files which have hard links?

[Dr-Emann]

Added a test which seems to verify we survive hard linking. However, it's probably a good idea to add an option (default on? or always?) to ignore files with hardlinks, since we'll necessarily break the hardlink when we replace the file with the compressed version, and that may be undesirable.