[MERGE] RemoteBranch uses inefficient revision_id_to_revno

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> >> So, what would be a good way to make RemoteBranch.revision_id_to_revno
> >> fast, without having to duplicate all of the code in Branch6/7?
> > 
> > It seems like the best options are:
> > - delegate to the underlying Branch
> 
> Done in attached.
> 
> Basically, this lets the different implementations use the best method.
> It is probably a small step backwards to avoiding VFS operations, but it
> will make *my* life with branches bound over the network a bit better.

I think this is an ok stop-gap.  An RPC for this would be good to have, but in
the meantime I see no reason not to do this.

bb:approve

-Andrew.