Bugfix: Correct hardlink handling in `stats` command

If files on different devices had the same inode ID, the `stats` command
did not correctly calculate the snapshot size. This has now been fixed.

https://forum.restic.net/t/possible-bug-in-stats/6461/8
https://github.com/restic/restic/pull/4503
https://github.com/restic/restic/pull/4006
