[MERGE] Stacked knits

[Robert Collins]

This patch teaches KnitVersionedFiles how to stack on any other
VersionedFiles object(s). It doesn't [yet] support annotations across
such boundaries, but as it should be sufficient to get stacked branches
up and running I thought I would send it in for review.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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