[MERGE] Graph.heads()

[Robert Collins]

Robert Collins has voted resubmit.
Status is now: Resubmit
Comment:
In packs we have a larger graph space.
Rather than one graph per file, we have one graph for the repository, 
with pairs (file_id, revision_id) as keys.

It would be nice to cut a couple of layers out and work directly in that 
space. If not, then at least the repository should be able to do that 
itself and translate only the result - so I'd really prefer to see this 
functionality on repository.

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