brisbane: initial cut at a mergeline cache

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> > Why would you be numbering revisions that aren't in your ancestry?  To
> > describe a pending merge?
[...]
> 
> Or stuff like "bzr qlog branch1 branch2", yes. Or if you wanted to
> consider dead heads in the repository, or ...

How does qlog label a revision that has different dotted revnos in branch1
vs. branch2?

I'd be happy to have non-ancestor revisions be displayed with some different
revision number scheme.  annotate already does this with uncommitted changes
(and maybe pending merges?), using a "?" in the revno.  Failing all else
there's the revid, of course...

-Andrew.