[MERGE] Repository.get_revision_graph improvements

[Ian Clatworthy]

John Arbash Meinel wrote:
> Aaron Bentley wrote:
> | John Arbash Meinel wrote:
> |> 2) Introduce Graph.iter_ancestry() which returns similar information.
> |
> | bb:resubmit

I've reviewed this and it looks good to me, except for a minor indenting
issue inside NEWS.

bb:tweak.

It would be really nice to land this for 1.3 if it speeds up log as I
understand it will. I don't pretend to know this area of the code that
well though, so it may be worth getting another review just to be safe.
Perhaps Aaron could double-check that graph.py and pack_repo.py are both
to his liking now?

Ian C.