show distribution mismatch for changed codenames

We have the last Release file around for other checks, so its trivial to
look if the new Release file contains a new codename (e.g. the user has
"testing" in the sources and it flipped from stretch to buster).

Such a change can be okay and expected, but also be a hint of problems,
so a warning if we see it happen seems okay. We can only print it once
anyhow and frontends and co are likely to ignore/hide it.

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