brisbane: initial cut at a mergeline cache

[Talden]

> Bazaar has already established a culture and an understanding amongst
> its users that the revno on each branch is just a shorthand specific to
> that branch. Frankly, I don't think anyone would notice if the dotted
> revnos went away

I would.  Knowing the number of 'things' that have landed in the
mainline is useful.  I wouldn't mind them being numbered from the
merged revision though.

--
Talden