show distribution mismatch for changed codenames

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.