Enhancement: Back up mountpoints as empty directories

When the `--one-file-system` option is specified to `restic backup`, it
ignores all file systems mounted below one of the target directories. This
means that when a snapshot is restored, users needed to manually recreate
the mountpoint directories.

Restic now backs up mountpoints as empty directories and therefore implements
the same approach as `tar`.

https://github.com/restic/restic/issues/909
https://github.com/restic/restic/pull/3119
