Enhancement: Include repository id in filesystem name used by `mount`

The filesystem created by restic's `mount` command now includes the repository
id in the filesystem name. The repository id is printed by restic when opening
a repository or can be looked up using `restic cat config`.

```
[restic-user@hostname restic]$ df ./test-mount/
Filesystem        1K-blocks  Used Available Use% Mounted on
restic:d3b07384d1         0     0         0    - /mnt/my-restic-repo
```

https://github.com/restic/restic/issues/4868
https://github.com/restic/restic/pull/5243