"Download as diff" is broken for an MR

Look at this: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1186/diffs

A neat one-line changeset. Now try "Download as diff" (same as clicking https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1186.diff). After some quite a churn, this produces a diff over 9000 lines long that wants to undo a lot of later changes to the master.

Looks like a symmetric difference ... operator was used somewhere instead of .. when calling git diff, but I am not qualified to find where the breakage occurs, sorry.