diff -r 6db38c9d7e00 -r cc3543c87de5 mercurial/revlog.py
--- a/mercurial/revlog.py	Fri Jun 22 00:05:20 2018 +0100
+++ b/mercurial/revlog.py	Fri Jun 22 00:07:22 2018 +0100
@@ -1404,7 +1404,8 @@
 
         The implementation of this is trivial but the use of
         commonancestorsheads is not."""
-        return a in self.commonancestorsheads(a, b)
+        a, b = self.rev(a), self.rev(b)
+        return self.descendant(a, b)
 
     def ancestor(self, a, b):
         """calculate the "best" common ancestor of nodes a and b"""