Update the Overview tab when the MR has finished async preparation

What does this MR do and why?

For #407115

Problem

The asynchronous preparation of MRs leads to an initial experience that indicates there are no changes - indefinitely - when there definitely are changes, they just haven't been prepared yet.

Fix

This MR hooks into the live subscription that will update when the MR has been prepared so that it can dynamically update the UI the moment we know it's ready.

Screenshots or screen recordings

Before	After
RecApp-2023-05-15-16_43_12	RecApp-2023-05-15-16_32_15

How to set up and validate locally

1. Add sleep(15.seconds) here
2. Create a new MR

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.