Bringing back the merge button in an MR's changes view
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### Problem to solve
When you're on an MR's `changes` view, you can't Merge. You need to navigate back before being able to do so
### Further details
In older versions you were able to do so, such as in `v11.11.3`:

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In the [current version](https://gitlab.com/nelbacha/changes-mr-button-test/-/merge_requests/1/diffs) that's not possible anymore:

### Proposal
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### What does success look like, and how can we measure that?
You can merge when you're on the `changes` view
issue