[git-cl] fix assertion.

We want to assert that if add_owners_to is provided, change is also
provided.

Bug:

Change-Id: I0b9674dcd1cdd5e42a8ab92583570ecc02ba1dcd
Reviewed-on: https://chromium-review.googlesource.com/481040
Reviewed-by: Daniel Jacques <dnj@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

git_cl.py

@@ -3262,7 +3262,7 @@ class ChangeDescription(object):
     assert isinstance(tbrs, list), tbrs
 
     assert add_owners_to in (None, 'TBR', 'R'), add_owners_to
-    assert not add_owners_to or not change, add_owners_to
+    assert not add_owners_to or change, add_owners_to
 
     if not reviewers and not tbrs and not add_owners_to:
       return