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 hook: Hook | null = currentlyRenderingFiber.memoizedState;
    while (hook !== null) {
      const queue = hook.queue;
      if (queue !== null) {
        queue.pending = null;
      }
      hook = hook.next;
    }
    didScheduleRenderPhaseUpdate = false;
  }
  renderLanes = NoLanes;
  currentlyRenderingFiber = (null: any);
  currentHook = null;
  workInProgressHook = null;
  if (__DEV__) {
    hookTypesDev = null;
    hookTypesUpdateIndexDev = -1;
    currentHookNameInDev = null;
  }
  didScheduleRenderPhaseUpdateDuringThisPass = false;
  localIdCounter = 0;
  thenableIndexCounter = 0;
  thenableState = null;
}
function mountWorkInProgressHook(): Hook {
  const hook: Hook = {