Fix branch matching logic

Based on Jim's feedback, change the branch matching logic to always
have priority over ref matching. And v3 will always have refs, so no
need to check if that attribute exists. Also adds a test that checks the
current breakage of branch matching logic.

Change-Id: Iba148b73a77b3300ad84db1c05c083d2c82cd950
diff --git a/tests/unit/test_change_matcher.py b/tests/unit/test_change_matcher.py
index 6b161a1..3d5345f 100644
--- a/tests/unit/test_change_matcher.py
+++ b/tests/unit/test_change_matcher.py
@@ -60,7 +60,7 @@
         self.assertTrue(self.matcher.matches(self.change))
 
     def test_matches_returns_true_on_matching_ref(self):
-        self.change.branch = 'bar'
+        delattr(self.change, 'branch')
         self.change.ref = 'foo'
         self.assertTrue(self.matcher.matches(self.change))
 
@@ -69,11 +69,6 @@
         self.change.ref = 'baz'
         self.assertFalse(self.matcher.matches(self.change))
 
-    def test_matches_returns_false_for_missing_attrs(self):
-        delattr(self.change, 'branch')
-        # ref is by default not an attribute
-        self.assertFalse(self.matcher.matches(self.change))
-
 
 class TestFileMatcher(BaseTestMatcher):