Fix: Optimistic update does not get reset (#27453)

I found a bug where if an optimistic update causes a component to
rerender, and there are no other state updates during that render, React
bails out without applying the update.

Whenever a hook detects a change, we must mark the component as dirty to
prevent a bailout. We check for changes by comparing the new state to
`hook.memoizedState`. However, when implementing optimistic state
rebasing, I incorrectly reset `hook.memoizedState` to the incoming base
state, even though I only needed to reset `hook.baseState`. This was
just a mistake on my part.

This wasn't caught by the existing tests because usually when the
optimistic state changes, there's also some other state that marks the
component as dirty in the same render.

I fixed the bug and added a regression test.