[MERGE] Graph.heads()

[Aaron Bentley]

Aaron Bentley has voted resubmit.
Status is now: Resubmit
Comment:
This code doesn't work, because you can't construct a Graph directly 
from a VersionedFile.  (You need a ParentsProvider, because 
VersionedFile's get_parents doesn't do what graph.Graph wants).  And 
it's not clear whether it can be implemented sanely on weaves, 
considering the way they handle ghosts.

That said, I do think that Graph.heads() makes sense, and that interface 
can be implemented efficiently on any Graph implementation.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46C2257B.4000307%40arbash-meinel.com%3E