Nested `startTransition` call "takes over" and makes parent `startTransition` unable to track `isPending`.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. `isPending` in `IndexPage.js` is `true`

However, `isPending` in `App.js` is `false`.

Expected: `isPending` in `App.js` is also `true`.
function updateState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  return updateReducer(basicStateReducer, (initialState: any));
}
function rerenderState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  return rerenderReducer(basicStateReducer, (initialState: any));
}
function pushEffect(
  tag: HookFlags,
  create: () => (() => void) | void,
  destroy: (() => void) | void,
  deps: Array<mixed> | void | null,
): Effect {
  const effect: Effect = {
    tag,
    create,
    destroy,
    deps,
    
    next: (null: any),
  };
  let componentUpdateQueue: null | FunctionComponentUpdateQueue =
    (currentlyRenderingFiber.updateQueue: any);
  if (componentUpdateQueue === null) {
    componentUpdateQueue = createFunctionComponentUpdateQueue();
    currentlyRenderingFiber.updateQueue = (componentUpdateQueue: any);
    componentUpdateQueue.lastEffect = effect.next = effect;