Missing diff from merge request

Summary

Merge request is missing diff

Steps to reproduce

See:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8248
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8248.json

Expected behavior

Diff exists 😕

Actual behavior

Diff doesn't exist 😕

Relevant logs and/or screenshots

MergeRequestDiff id: 1986187, state: "empty", st_commits: nil, st_diffs: [], merge_request_id: 1580366, created_at: "2016-12-22 15:27:03", 
updated_at: "2016-12-22 15:27:28", base_commit_sha: "4b7395e0fab8e35436957e41929b44f948a1c41a", real_size: nil, head_commit_sha: 
"4b7395e0fab8e35436957e41929b44f948a1c41a", start_commit_sha: "fbd8fd883bfc1c1c6e5fda132c8759c608fa7980">

@zj @markglenfletcher @stanhu