fix gitlab-org/gitlab-ee#2190

What does this MR do?

fix gitlab-org/gitlab-ee#2190

When you get raw by http://gitlab.example.com/api/v4/projects/user%2Fproj/repository/files/README.md/raw?ref=master, got 404 not found.

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

no, it's simple. I think.

Why was this MR needed?

http://gitlab.example.com/api/v4/projects/user%2Fproj/repository/files/README.md/raw?ref=master returns 404. Developer cannot get raw file content directly.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• 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?