Fix the /projects/:id/repository/commits endpoint to handle dots in the ref name…

What does this MR do?

Part 3 of #15651 (closed). This allows to make API requests for refs that contains a . when the project full path contains a /.

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

Same as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13115/diffs#are-there-points-in-the-code-the-reviewer-needs-to-double-check.

Why was this MR needed?

Because refs that contain a . couldn't be dealt with when the project full path contains 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 Backend
• 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

Fixes #15651 (closed).