[MERGE] repository external reference tests (StackedBranches)

[Martin Pool]

At Ian's request I'm going to do these tweaks and submit it.

I'm going to rename the module to per_repository_reference which is
consistent with at least per_lock and much more wieldy.

I do note that at the moment these tests are not run because no
formats that support external references are currently registered.
And that points out for me another advantage of separating generation
of scenarios from loading tests, that we can test that pure function.

-- 
Martin <http://launchpad.net/~mbp/>