Better parent support

[aawsome]

rustic currently chooses the latest snapshot with identical host and path(s) (as used by the backup command) as parent.

This can be improved:

• include snapshots which have "super-paths", like allow /home,/usr as parent for /home or allow /home as parent for /home/bob.
• include sub-paths if there is no other parent candidate or if these are younger than other snapshots. This might imply to use multiple snapshots as parent, though.

[aawsome]

see also restic/restic#3121