Fix issues importing forked projects

Fixes 500 errors on forked projects.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/26184.

Also, more importantly, fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/29380

As all merge request diffs stopped working and no diff/commits were shown.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together