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 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.

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