assertion: a repo has either unstable and unstable_xenial or unstable and unstable_bionic but never unstable and unstable_xenial and unstable_bionic. This needs actual validation!

This is a prerequisite for the branch reshuffle. Neon/unstable would either be bionic default or xenial default, as such there should be no repos where all three variants appear as its then not clear which is actually the wanted repo.