[MERGE] updated index bisection

[Martin Pool]

> > +    def lookup_keys_via_location(self, location_keys):
> > +        """Public interface for implementing bisection.
> >
> > Is this intended to be a public interface?  It looks more like a special
> > callback for bisection and so it shouldn't be public.
>
> Well, I really prefer to be testing public interfaces, and as the
> bisection interface is a defined one, this is in my mind public. I'm
> happy to make it non public if you still think thats better after
> reading this.

I agree in general testing public interfaces is better.  Obviously in
some systems you can _only_ practically test public interfaces, which
is not the case in Python, but there are other reasons.

The issue of just what public/private means has come up before.  I
think in Bazaar the most important distinction, and the one we should
mostly use _prefix for, is interfaces that are not promised to be
stable in the long term, or that plugins can/should use.