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`.
hook.memoizedState = value;
    return value;
  }
}
function startTransition(
  setPending: boolean => void,
  callback: () => void,
  options?: StartTransitionOptions,
): void {
  const previousPriority = getCurrentUpdatePriority();
  setCurrentUpdatePriority(
    higherEventPriority(previousPriority, ContinuousEventPriority),
  );
  setPending(true);
  const prevTransition = ReactCurrentBatchConfig.transition;
  ReactCurrentBatchConfig.transition = ({}: BatchConfigTransition);
  const currentTransition = ReactCurrentBatchConfig.transition;
  if (enableTransitionTracing) {
    if (options !== undefined && options.name !== undefined) {
      ReactCurrentBatchConfig.transition.name = options.name;
      ReactCurrentBatchConfig.transition.startTime = now();
    }
  }
  if (__DEV__) {
    ReactCurrentBatchConfig.transition._updatedFibers = new Set();
  }
  try {
    setPending(false);
    callback();
  } finally {