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`.
type MutableSourceMemoizedState<Source, Snapshot> = {
  refs: {
    getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
    setSnapshot: Snapshot => void,
  },
  source: MutableSource<any>,
  subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
};
function readFromUnsubscribedMutableSource<Source, Snapshot>(
  root: FiberRoot,
  source: MutableSource<Source>,
  getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
): Snapshot {
  if (__DEV__) {
    warnAboutMultipleRenderersDEV(source);
  }
  const getVersion = source._getVersion;
  const version = getVersion(source._source);
   Is it safe for this component to read from this source during the current render?
  let isSafeToReadFromSource = false;
   Check the version first.
   If this render has already been started with a specific version,
   we can use it alone to determine if we can safely read from the source.
  const currentRenderVersion = getWorkInProgressVersion(source);
  if (currentRenderVersion !== null) {
    
    
    isSafeToReadFromSource = currentRenderVersion === version;
  } else {