[MERGE] Clarify tests in revisionstore_implementations/test_all

[Robert Collins]

Actually, I've thought about this more, asking for a revision None is
nuts; giving 'NoSuchRevision' for None, while true, is not really
helpful - TypeError, which is what the check_revision_id call raises is
better.

So I propose to change it to wanting TypeError for None.

ok?

-Rob
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