Ignore merge point candidates that create crosses

In OBS you can create submit requests for revisions that are behind
the last merge point, in git you can't - so we ignore them.

Fixes #14

tests/tree_test.py

@@ -71,5 +71,6 @@ class TestTreeMethods(unittest.TestCase):
     def test_firewalld_tree(self):
         self.verify_package("firewalld")
 
+
 if __name__ == "__main__":
     unittest.main()