diff -r 160da69ba1bf -r 6afa928033bd mercurial/revlog.py
--- a/mercurial/revlog.py	Wed Jul 11 16:21:41 2018 -0700
+++ b/mercurial/revlog.py	Wed Jul 11 16:37:30 2018 -0700
@@ -1656,6 +1656,8 @@
             return True
         elif a == b:
             return True
+        elif a < b:
+            return False
         return b in self._commonancestorsheads(a, b)
 
     def commonancestorsheads(self, a, b):