revlog: reuse 'descendant' implemention in 'isancestor'

The two functions do the same thing, but one takes nodes while the other takes
revs. Using one to implement the other make sense.

We should probably cleanup the API at some point to avoid having so many similar
functions. However, we focus on an efficient implementation for now.

mercurial/revlog.py

@@ -1404,7 +1404,8 b' class revlog(object):'
 
         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"""