update should lookup parent first

[fsoikin]

According to Enzyme docs, update only works on the root element, and calling it on a non-root element doesn't crash, but has no effect.

This is a bit of a hole in our API (and in Enzyme's own API as well), because once you call update (or waitUntil) inside a withSelector, it's very unobvious why the bloody thing isn't working. Caused me a 20-minute debugging session once.

To mitigate this, I think the most ergonomic way is to have the update function take care of looking up the parent on its own.

At one point I tried the naive approach of calling parent in a loop, but that doesn't quite work: the result never becomes null, but also never becomes equal to itself for some reason. I did not investigate further at the time, and chose to abandon the effort.