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`.
deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useImperativeHandle';
      updateHookTypesDev();
      return updateImperativeHandle(ref, create, deps);
    },
    useInsertionEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useInsertionEffect';
      updateHookTypesDev();
      return updateInsertionEffect(create, deps);
    },
    useLayoutEffect(
      create: () => (() => void) | void,
      deps: Array<mixed> | void | null,
    ): void {
      currentHookNameInDev = 'useLayoutEffect';
      updateHookTypesDev();
      return updateLayoutEffect(create, deps);
    },
    useMemo<T>(create: () => T, deps: Array<mixed> | void | null): T {
      currentHookNameInDev = 'useMemo';
      updateHookTypesDev();
      const prevDispatcher = ReactCurrentDispatcher.current;
      ReactCurrentDispatcher.current =
        InvalidNestedHooksDispatcherOnUpdateInDEV;
      try {
        return updateMemo(create, deps);