dillon1000/react

Don't use existing "root" ID if non-root

Before, if you had

```
container = <div data-reactid=".0"><div data-reactid=".0.0" /></div>;
```

and did `ReactDOM.render(<span />, container)` you would get

```
<div data-reactid=".0"><span data-reactid=".0" /></div>;
```

(along with a warning not to replace React-rendered children with a new tree like that). But that makes no sense -- the span should have a new index, not truncate the ID of the old child it's replacing.

(Now tests pass again with useCreateElement on; before they threw a "valid but unequal" on our test for this warning.)