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`.
}
  let nextWorkInProgressHook: null | Hook;
  if (workInProgressHook === null) {
    nextWorkInProgressHook = currentlyRenderingFiber.memoizedState;
  } else {
    nextWorkInProgressHook = workInProgressHook.next;
  }
  if (nextWorkInProgressHook !== null) {
    
    workInProgressHook = nextWorkInProgressHook;
    nextWorkInProgressHook = workInProgressHook.next;
    currentHook = nextCurrentHook;
  } else {
    
    if (nextCurrentHook === null) {
      const currentFiber = currentlyRenderingFiber.alternate;
      if (currentFiber === null) {
        
        
        
        throw new Error(
          'Update hook called on initial render. This is likely a bug in React. Please file an issue.',
        );
      } else {
        
        throw new Error('Rendered more hooks than during the previous render.');
      }
    }
    currentHook = nextCurrentHook;
    const newHook: Hook = {