Don't save digests in the playlist file.

They create dependency loops:
  digest_{i+1} = checksum(file with digest_i)
With the old way, exiting after
  $ hk.py -c 'new_playlist --output_playlist mylist'
          -c 'glob_curves_to_playlist ...*.curve'
          -c 'save_playlist --output mylist' -p
would complain of unsaved playlists.