Improve MTR differ

[lunakv]

The MTR differ is currently in a "good enough" state where manual involvement tends to be minimal, but there are still areas where it's sub-par, mostly when handling changes in lists. This tends to manifest most readily in lists of available sets, where one update will remove the effective date of an item and add new items with their effective dates at the same time, which causes weird overlaps that have to be fixed manually. We should think about ways to mitigate this behavior.