Bugfix: Fix tar issues when dumping `/`

We've fixed an issue with dumping either `/` or files on the first sublevel
e.g. `/foo` to tar. This also fixes tar dumping issues on Windows where this
issue could also happen.

https://github.com/restic/restic/issues/2254
https://github.com/restic/restic/issues/2357
https://github.com/restic/restic/pull/2255
