[MERGE] Add support for previewing the result of a merge

[Michael Hudson]

Aaron Bentley wrote:
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> Hi all,
> 
> This patch adds support for previewing the result of a merge or other
> TreeTransform.  This is a result of the combined efforts of me and
> Michael Hudson.  He wants it for Launchpad, but I can also see it being
> useful for Bundle Buggy, Loggerhead, or a graphical merge tool like John
> Meinel's merge-into-gui.

Yay, thanks so much for putting the effort in to getting this mergeable!

In my cold-inhibited fuddle, I didn't see a test for previewing a merge 
that deleted a file, and I _think_ I found a bug relating to this in my 
code a week or so ago... could it still be there?

Cheers,
mwh