Bugfix: Exclude irregular files from backups

Since restic 0.17.1, files with the type `irregular` could mistakenly be included
in snapshots, especially when backing up special file types on Windows that
restic cannot process. This issue has now been fixed.

Previously, this bug caused the `check` command to report errors like the
following one:

```
  tree 12345678[...]: node "example.zip" with invalid type "irregular"
```

To repair affected snapshots, upgrade to restic 0.17.2 and run:

```
restic repair snapshots --forget
```

This will remove the `irregular` files from the snapshots (creating
a new snapshot ID for each of the affected snapshots).

https://github.com/restic/restic/pull/5057
https://forum.restic.net/t/errors-found-by-check-1-invalid-type-irregular-2-ciphertext-verification-failed/8447/2
