Follow-up from "Use the API to retrieve a MR diff and fallback to Git"

The following discussion from !24 (merged) should be addressed:

@rymai started a discussion:
I realized we could use mr_changes here! See https://github.com/danger/danger/blob/108e82ab793cc48a2756b9881a94e01cce57d353/lib/danger/request_sources/gitlab.rb#L34.
```
      @changes_from_api ||= gitlab_helper.mr_changes.to_h["changes"]
```

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