storage/reflink: add revision even if empty

It's sort of useful to be able to revert a volume that has only ever
been started once to its empty state. And the lvm_thin driver allows it
too, so why not.

qubes/storage/reflink.py

@@ -184,8 +184,6 @@ class ReflinkVolume(qubes.storage.Volume):
     def _add_revision(self):
         if self.revisions_to_keep == 0:
             return
-        if _get_file_disk_usage(self._path_clean) == 0:
-            return
         ctime = os.path.getctime(self._path_clean)
         timestamp = qubes.storage.isodate(int(ctime))
         _copy_file(self._path_clean,