Reduce method calls while evaluating Projects::MergeRequestsController#show.json

What does this MR do?

Reduce method calls and general data being sent on when requesting Projects::MergeRequestsController#show.json.

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

What are the relevant issue numbers?

Part of #36876 (closed)