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