Fix N+1 issue for CompareController

What does this MR do and why?

Contributes to #360578 (closed)

Problem

1. We trigger N+1 queries for each commit to fetch pipeline status assigned to it.
2. We trigger N+1 queries for each commit to load markdown cache.

Solution

Preload pipeline statuses and markdown caches to avoid unnecessary DB and Redis queries.

Database

Before (N queries for each commit)

SELECT DISTINCT ON (sha) * FROM "ci_pipelines" WHERE "ci_pipelines"."project_id" = 1 AND ("ci_pipelines"."source" IN (1, 2, 3, 4, 5, 6, 7, 8, 10, 11) OR "ci_pipelines"."source" IS NULL) AND "ci_pipelines"."sha" = '2d1db523e11e777e49377cfb22d368deec3f0793' AND "ci_pipelines"."ref" = 'master' ORDER BY "ci_pipelines"."sha" ASC, "ci_pipelines"."id" DESC /*application:web,correlation_id:01G3XR8BFZ743XDVT4RVSCW5WG,endpoint_id:Projects::CompareController#show,db_config_name:ci,line:/app/models/ci/pipeline.rb:429:in `latest_pipeline_per_commit'*/

SELECT DISTINCT ON (sha) * FROM "ci_pipelines" WHERE "ci_pipelines"."project_id" = 1 AND ("ci_pipelines"."source" IN (1, 2, 3, 4, 5, 6, 7, 8, 10, 11) OR "ci_pipelines"."source" IS NULL) AND "ci_pipelines"."sha" = 'ddd0f15ae83993f5cb66a927a28673882e99100b' AND "ci_pipelines"."ref" = 'master' ORDER BY "ci_pipelines"."sha" ASC, "ci_pipelines"."id" DESC /*application:web,correlation_id:01G3XR8BFZ743XDVT4RVSCW5WG,endpoint_id:Projects::CompareController#show,db_config_name:ci,line:/app/models/ci/pipeline.rb:429:in `latest_pipeline_per_commit'*/ 

After (a single query)

SELECT DISTINCT ON (sha) * FROM "ci_pipelines" WHERE "ci_pipelines"."project_id" = 1 AND ("ci_pipelines"."source" IN (1, 2, 3, 4, 5, 6, 7, 8, 10, 11) OR "ci_pipelines"."source" IS NULL) AND "ci_pipelines"."sha" IN ('2d1db523e11e777e49377cfb22d368deec3f0793', 'ddd0f15ae83993f5cb66a927a28673882e99100b') AND "ci_pipelines"."ref" = 'master' ORDER BY "ci_pipelines"."sha" ASC, "ci_pipelines"."id" DESC /*application:web,correlation_id:01G3XR71VYRFBB6XWVMTXPGPZN,endpoint_id:Projects::CompareController#show,db_config_name:ci,line:/app/models/ci/pipeline.rb:429:in `latest_pipeline_per_commit'*/

Screenshots or screen recordings

Before	After

How to set up and validate locally

Visit http://127.0.0.1:3000/gitlab-org/gitlab-test/-/compare/1a0b36b3cdad1d2ee32457c102a8c0b7056fa863...master?from_project_id=1

MR acceptance checklist

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