Bugfix: The `stats` command miscalculated restore size for multiple snapshots

Since restic 0.10.0 the restore size calculated by the `stats` command for
multiple snapshots was too low. The hardlink detection was accidentally applied
across multiple snapshots and thus ignored many files. This has now been fixed.

https://github.com/restic/restic/issues/3736
https://github.com/restic/restic/pull/3740
