Fix diffs with edit-forking needs

What does this MR do?

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1705/

• Fix MR diffs where the user is required to fork in order to "Edit" a file

MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10602 caused https://gitlab.com/gitlab-org/gitlab-ce/issues/31276

Are there points in the code the reviewer needs to double check?

• When you don't have permissions to edit directly and need a fork, when you already have a fork, and try to edit a file, it will ask you to fork again and error out, https://gitlab.com/gitlab-org/gitlab-ce/issues/31281
  • This bug was also present before this new fork/cancel confirmation, blob_helper -> edit_blob_link

Why was this MR needed?

• When viewing a MR from fork (from another use other than author), the page would 500, https://gitlab.com/gitlab-org/gitlab-ce/issues/31276

Does this MR meet the acceptance criteria?

• 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

What are the relevant issue numbers?

Closes #31276 (closed)