Fix Diff::Position#diff_file for positions on straight diffs
Fixes root cause for the link in https://dev.gitlab.org/gitlab/version-gitlab-com/merge_requests/72#note_104465 not having an anchor pointing to exact line that changed.
Bug surfaced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11584, which is a 9.3 feature, which makes this a regression!