Reduce queries in spec/requests/api/merge_requests_spec.rb

What does this MR do?

This reduces the queries from 97137 to 65136 and the query time from 02:04 to 01:27 in spec/requests/api/merge_requests.rb.

What are the relevant issue numbers?

No issue number, this file is one of the main offenders in the "# of queries" category here: https://redash.gitlab.com/dashboard/test-suite-statistics

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer