Cannot use the mountpoint of subvolumes as a subvolume

In the root subvolume there are three subvolumes: @, @home, and @snapshots.
@ is mounted as /, @home is mounted as /home, and @snapshots is mounted as both /.snapshots and /home/.snapshots.

I would like to be able to set up the btrbk configuration like this

```
volume ssh://planeptune/
subvolume .
target /mnt/backups/remote/planeptune/root
subvolume home
target /mnt/backups/remote/planeptune/home
```

But if I do I get this warning

WARNING: Skipping subvolume "planeptune:/./home": Not a child subvolume of: planeptune:/

This forces me to use the following in my configuration which is less than optimal since it forces me to define multiple volumes when really one should suffice

```
volume ssh://planeptune/
subvolume .
target /mnt/backups/remote/planeptune/root

volume ssh://planeptune/home
subvolume .
target /mnt/backups/remote/planeptune/home
```