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`.
if (currentUpdateQueue !== null) {
        const currentMemoCache: ?MemoCache = currentUpdateQueue.memoCache;
        if (currentMemoCache != null) {
          memoCache = {
            data: currentMemoCache.data.map(array => array.slice()),
            index: 0,
          };
        }
      }
    }
  }
   Finally fall back to allocating a fresh instance of the cache
  if (memoCache == null) {
    memoCache = {
      data: [],
      index: 0,
    };
  }
  if (updateQueue === null) {
    updateQueue = createFunctionComponentUpdateQueue();
    currentlyRenderingFiber.updateQueue = updateQueue;
  }
  updateQueue.memoCache = memoCache;
  let data = memoCache.data[memoCache.index];
  if (data === undefined) {
    data = memoCache.data[memoCache.index] = new Array(size);
    for (let i = 0; i < size; i++) {
      data[i] = REACT_MEMO_CACHE_SENTINEL;
    }
  } else if (data.length !== size) {