Fix regex to allow proper branch naming.

What does this MR do?

Fix regex to allow proper branch names

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

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12198#note_37134444

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Review
  • Has been reviewed by Frontend
• 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?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12198#note_37134444