Bugfix: Correctly handle passing volume name to `backup` command

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 now works correctly.

To restore snapshots created before this bugfix, use the `<snapshot>:<subpath>`
syntax. For a snapshot with ID `12345678` and a backup of `C:`, the following
command can be used:

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

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