[MERGE] [BUG #172970] bzrlib.revision.is_ancestor should use graph functions.

[Robert Collins]

Robert Collins has voted approve.
Status is now: Approved
Comment:
It would be nice if we could use heads() here. I think a call 
'reachability' that would look like this:

+   graph = self.this_branch.repository.get_graph()
+   reachables = graph.reachables([(self.base_rev_id, self.this_basis),
  (self.base_rev_id, self.other_basis)])

Or something.

Another possibility is that we can learn by using heads - anyhow, as a 
direct replacement this is fine.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C4750340D.9080201%40arbash-meinel.com%3E