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`.
state = {
          label: 'A',
          contents: 'A',
        };
        render() {
          update = value => {
            ReactNoop.discreteUpdates(() => {
              this.setState({label: value});
              startTransition(() => {
                this.setState({contents: value});
              });
            });
          };
          const label = this.state.label;
          const contents = this.state.contents;
          const isContentPending = label !== contents;
          return (
            <>
              <Text
                text={
                  label + ' label' + (isContentPending ? ' (loading...)' : '')
                }
              />
              <div>
                <Suspense fallback={<Text text="Loading..." />}>
                  <AsyncText text={contents + ' content'} />
                </Suspense>
              </div>