Add possibility to download .patch/.diff from merge request versions
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=27447) </details> <!--IssueSummary end--> GitLab stores merge request internally, but there's no way to retrieve these. Example, i've tried: * `https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22330/diffs?diff_id=25004746.patch` * `https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22330/diffs?diff_id=25004746.diff` * `https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22330.diff?diff_id=25004746` * `https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22330.patch?diff_id=25004746` See context: - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22330#note_147364972
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