API: Expose branch status

What does this MR do?

Expose the merge status of a branch,

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

For the default branch, the merge status is always false. Another possibility would be to conditionally expose the merge status only for branches not equal to the default branch.

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 https://gitlab.com/gitlab-org/gitlab-ce/issues/24938