assert_one_change() would fail if it detects that HEAD exists on the
remote (on any branch at all). However, some gerrit projects are
configured to allow such reviews to be opened so long as HEAD does not
exist on the target branch. Start warning rather than failing and let
the gerrit server do its own check.
Story: 2010887
Task: 48652
Change-Id: I5040aa24d78abec31054d7eeee9f6f27ce538988