Fix duplicate item in protected branch/tag dropdowns

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4137

• Fix duplicate item in protected branch/tag dropdowns
• Add some tests

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

Steps to reproduce

1. Visit the project settings -> Repository page and see the "Protected Branches" section

2. Enter the name of a branch that already appears in the dropdown list

3. Click the "Create wildcard ..." button

4. Open the dropdown back up and see the duplicate

https://gitlab.com/gitlab-org/gitlab-ce/issues/42157

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #42157 (closed) https://gitlab.com/gitlab-org/gitlab-ce/issues/41989