Further reduce N+1 SQL queries on mergeable/detailedMergeStatus fields

Background

Based on #437975 (comment 1765459339), after drastically reducing the SQL queries, there are still N+1 queries when mergeable or detailedMergeStatus graphQL fields are requested for multiple MRs.

Proposal

Look into queries shared in #437975 (comment 1765459985) and improve them.