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`.
expect(root).toMatchRenderedOutput('Async');
  });
  
  test(
    'when multiple transitions update the same queue, only the most recent ' +
      'one is allowed to finish (no intermediate states)',
    async () => {
      let update;
      function App() {
        const [isContentPending, startContentChange] = useTransition();
        const [label, setLabel] = useState('A');
        const [contents, setContents] = useState('A');
        update = value => {
          ReactNoop.discreteUpdates(() => {
            setLabel(value);
            startContentChange(() => {
              setContents(value);
            });
          });
        };
        return (
          <>
            <Text
              text={
                label + ' label' + (isContentPending ? ' (loading...)' : '')
              }
            />
            <div>
              <Suspense fallback={<Text text="Loading..." />}>
                <AsyncText text={contents + ' content'} />