Code review comment for lp:~al-maisan/launchpad/subsets

Hello there!

The branch at hand fixes bug #472326. When a package set P is to add/remove
another package (sub-)set Q then both P and Q must be related to the same
distro series.

The bug was discovered during testing on Soyuz dogfood and is
release-critical.

Tests to run:

    bin/test -vv -t packageset.txt