lvm_thin: _remove_revisions() on revisions_to_keep==0

If revisions_to_keep is 0, it may nevertheless have been > 0 before, so
it makes sense to call _remove_revisions() and hold back none (not all)
of the revisions in this case.

qubes/storage/lvm.py

@@ -262,7 +262,8 @@ class ThinVolume(qubes.storage.Volume):
         if revisions is None:
             revisions = sorted(self.revisions.items(),
                 key=operator.itemgetter(1))
-            revisions = revisions[:-self.revisions_to_keep]
+            # pylint: disable=invalid-unary-operand-type
+            revisions = revisions[:(-self.revisions_to_keep) or None]
             revisions = [rev_id for rev_id, _ in revisions]
 
         for rev_id in revisions: