brisbane: initial cut at a mergeline cache

[Aaron Bentley]

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Matthew D. Fuller wrote:
> I've preferred merge-point-based numbering since we had the
> dicussion when dotted revnos were being implemented.

+1.

> It also has other advantages; picture reading through a long --long
> (well, with -n0 now) on something like bzr.dev, where each mainline
> rev is a merge of dozens or hundreds of revs.  With merge-point-based
> numbering, you know about where you are in the history.  With
> ancestral, you don't, and if you're flipping through it fast you can
> even blow right past the next (previous) mainline rev without even
> noticing it.

My primary use case for log --long -n1 is finding which revision merged
another revision, so merge-point numbering would help me avoid that
entirely.

Aaron
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