cmd_merge is too long, and needs tests

[Gavin Panella]

There's a lot of critically import logic in _do_merges. I think it ought to be factored out into testable units. And tested :)

[dobey (dobey)]

Hrmm. I just poked through these two branches that are linked, and the amount of invasive change is a bit offputting. The changes are very large, and i think it's slightly the wrong way to go about testing the merging code that pokes launchpad.

[Gavin Panella (allenap)]

Rodney, I agree. I realised after getting this far that I should have done TDD on a new implementation of the merge code, using the existing code as a recipe book, or something like that. The ProposalMerger class was an attempt to break up the code into some testable chunks, but it is a bit clunky.

I did this work for Landscape, to get the various landing targets handled by Tarmac. I've moved back to the Launchpad team now, so I've handed the work back to Landscape. Bjorn was involved in this, and I've subscribed him to this bug. I don't know if he'll continue work on my branches or start afresh, or even if he has time to work on this bug at all.