[MERGE] Some external reference tests.

[Robert Collins]

This adds some of the full collection of external reference repository
tests. They are a specialised set of tests for testing the behaviour of
a repository which only contains partial data of some history.

This set happens to be enough to test most of the repository api, and
I'm now focusing on implementing enough that they all pass; I'll come
back to the rest of the Repository API when I have the check test
passing. I'd like to merge this now, because it keeps the overall branch
size down.

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

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