Investigate and fix incorrect N + 1 query specs in Merge request domain
MR: Pending
Description
Spin off from: !219362 (comment 3035659027)
While attempting to fix an observed performance regression in Stop preloading diff commits (!219362), we identified N + 1 queries in global search merge request lookups. These N + 1 queries apparently always existed, but were skipped over in the specs due to setup differences ( the specs need Elasticsearch enabled on pipelines to pass, but this is not a default due to setup costs); a threshold was added to allow the specs to pass as is.
We also found out that some specs on the merge request API silently allowed N + 1 lookups to pass due to incorrect setup. These specs were rewritten, and a threshold was added to allow existing code to pass.
This issue aims to provide context to thresholds placed in the code, and as a reference point to either fix the N + 1 queries, or to link to specs where a threshold was needed due to the original setup to catch these queries not being correct