#826 introduced the concept of a magic trailing comma: if you, the programmer, explicitly put a trailing comma in a collection, it signals to Black that you want a given collection exploded into multiple lines, always. While this flies a bit in the way of "doesn't look at pre-existing formatting" and "non-configurability", it is extremely pragmatic and consistent with "make diffs as small as possible".

However, there's issues with it so far. It doesn't yet work on nested collections. Worse yet, in those cases it leaves trailing commas behind when a collection ends up compressed into one line. We need to fix those edge cases to make it generally dependable.