With snapper 0.13.1 the snapshot numbers should be all-time unique in
that even after deleting a snapshot the number is not reused for new
snapshots.

This is basically done by 1. keeping the directory if the snapshot
with the highest number is removed and 2. also looking at empty
directories when finding the number for the next snapshot.

As a consequence there can temporarily be empty directories in the
infos directories.
