Make bzrlib.graph._StackedParentsProvider api public

[Andrew Bennetts]

Gary van der Merwe wrote:
> Hi
> 
> So I'm using bzrlib.graph._StackedParentsProvider in qlog when viewing
> branches from different repositories to union the graphs.
> 
> It would be nice if this could be made into a public api. What needed
> to make this happen?

A patch to rename it.  It should probably include some unit tests if there
aren't already some.

-Andrew.