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`.
if (lastRenderPhaseUpdate !== null) {
    
    queue.pending = null;
    const firstRenderPhaseUpdate = lastRenderPhaseUpdate.next;
    let update = firstRenderPhaseUpdate;
    do {
      
      
      
      const action = update.action;
      newState = reducer(newState, action);
      update = update.next;
    } while (update !== firstRenderPhaseUpdate);
     Mark that the fiber performed work, but only if the new state is
     different from the current state.
    if (!is(newState, hook.memoizedState)) {
      markWorkInProgressReceivedUpdate();
    }
    hook.memoizedState = newState;
     Don't persist the state accumulated from the render phase updates to
     the base state unless the queue is empty.
     TODO: Not sure if this is the desired semantics, but it's what we
     do for gDSFP. I can't remember why.
    if (hook.baseQueue === null) {
      hook.baseState = newState;
    }
    queue.lastRenderedState = newState;
  }
  return [newState, dispatch];
}