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 numberOfReRenders: number = 0;
  let children;
  do {
    if (didScheduleRenderPhaseUpdateDuringThisPass) {
      
      
      thenableState = null;
    }
    thenableIndexCounter = 0;
    didScheduleRenderPhaseUpdateDuringThisPass = false;
    if (numberOfReRenders >= RE_RENDER_LIMIT) {
      throw new Error(
        'Too many re-renders. React limits the number of renders to prevent ' +
          'an infinite loop.',
      );
    }
    numberOfReRenders += 1;
    if (__DEV__) {
      
      
      ignorePreviousDependencies = false;
    }