Bugfix: Correctly handle volume names in `backup` command on Windows

On Windows, when the specified backup target only included the volume
name without a trailing slash, for example, `C:`, then restoring the
resulting snapshot would result in an error. Note that using `C:\`
as backup target worked correctly.

Specifying volume names is now handled correctly. To restore snapshots
created before this bugfix, use the <snapshot>:<subpath> syntax. For
example, to restore a snapshot with ID `12345678` that backed up `C:`,
use the following command:

```
restic restore 12345678:/C/C:./ --target output/folder
```

https://github.com/restic/restic/issues/2004
https://github.com/restic/restic/pull/5028
