Resolve "branch names ending in .txt is not accessible via the api"

What does this MR do?

Correctly returns the api/v3/projects/:id/repository/branches/:branch where :branch is the name of a branch that ends in .txt such as other-feature.txt.

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

Unintended side effects ... this is a small change and I have not been able to reason why this is set. That being said this is my first pull-request to gitlab ...

Why was this MR needed?

It has been marked as a bug in Issue #23246 (moved)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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?

Closes #23246 (moved)