Fix #15651 - 404 in the API when accessing repository with a dot

What does this MR do?

This is a partial fix for https://gitlab.com/gitlab-org/gitlab-ce/issues/15651

It allow to access branch with the API when the repo got a dot in his name.

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

I didn't add a Changelog entry as there may be other fix needed to fully handle repository with dot (example: protect/unprotect branch, etc)

Why was this MR needed?

There was an old bug in the API.

Screenshots (if relevant)

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
• 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?

Partial fix #15651 (closed)