Remove many N+1 queries with merge requests API

Identified via `ENABLE_BULLET=1 bundle exec rspec spec/requests/api/merge_requests_spec.rb:34`

Improves speed of #34159

app/models/merge_request.rb

@@ -14,7 +14,7 @@ class MergeRequest < ActiveRecord::Base
   has_many :merge_request_diffs
   has_one :merge_request_diff,
-    -> { order('merge_request_diffs.id DESC') }
+    -> { order('merge_request_diffs.id DESC') }, inverse_of: :merge_request
   belongs_to :head_pipeline, foreign_key: "head_pipeline_id", class_name: "Ci::Pipeline"

lib/api/merge_requests.rb

@@ -41,7 +41,9 @@ module API
           args[:milestone_title] = args.delete(:milestone)
           args[:label_name] = args.delete(:labels)
-          merge_requests = MergeRequestsFinder.new(current_user, args).execute.inc_notes_with_associations
+          merge_requests = MergeRequestsFinder.new(current_user, args).execute
+                             .inc_notes_with_associations
+                             .preload(:target_project, :author, :assignee, :milestone, :merge_request_diff)
           merge_requests.reorder(args[:order_by] => args[:sort])
         end