Return more consistent values for merge_status on MR API

Check and update merge_status before presenting MRs on V4 API.

Reviewer needs to double-check

How heavy might these calls impact Gitaly services?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• 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 https://gitlab.com/gitlab-org/gitlab-ce/issues/20639