Fix the diff in the merge request view when converting a symlink to a regular file

What does this MR do?

It fixes a bug described in #21610 (closed)

Why was this MR needed?

There was a regression introduced in https://gitlab.com/gitlab-org/gitlab-ce/commit/cd7c2cb6ddd4d9c9f9bdae00c887c0022c121c17

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • 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?

Fixes #21610 (closed)