Add possibility to download .patch/.diff from merge request versions
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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
issue