Reduce diff with EE in ProtectedRefsController

What does this MR do?

This adds code missing from app/controllers/projects/protected_refs_controller.rb. EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8896

Somehow, EE is different than CE since https://gitlab.com/gitlab-org/gitlab-ee/commit/9ae15a938ddf (the corresponding CE change was f16377e7.

What are the relevant issue numbers?

Related to &27 (closed).

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer