Source interpol compatibility uses default source as reference

[justvanrossum]

#813 implements a "source compatibility" check in the form of a column in the source list. This uses the default source as reference. But this means that if the problem is in the default source, all other sources show the "bug".

This could be made better if we could efficiently find a better reference source. Ideally, the one that is part of the largest group of compatible sources.