Fix missing midpoint after undo (#2040)

See issue in #2040: when a segment is split by moving a midpoint, then the move is undone, the segment is left selected but no midpoint is visible.

The previous filter for determining which midpoints to redraw did not handle the case where a node was removed and the rejoined segment needed its midpoint drawn. I have added an extra condition to draw midpoints related to any nodes if its parent way that has changed.

I would love help writing a test for this but had quite a hard time since the code to test is several layers removed from its effect. I couldn't even figure out how to thread a spy in to it.