Use simple path for tmp ref, avoiding extra directory

What does this MR do?

Use simple path for tmp ref, avoiding extra directory.

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

None that I could think of. I don't think the path matter. Everything should still work.

Why was this MR needed?

So that we don't have to worry about cleaning empty directories later. This was brought up at:

https://gitlab.com/gitlab-org/gitlab-ce/issues/38689#note_42242988

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/38689#note_42242988