Prevent N+1 when diffStats is queried via GraphQL

Preload the `latest_merge_request_diff` when `diffStats` field
is queried by using lookahead.

Changelog: performance

app/graphql/resolvers/concerns/resolves_merge_requests.rb

@@ -59,7 +59,8 @@ def preloads
       timelogs: [:timelogs],
       pipelines: [:merge_request_diffs], # used by `recent_diff_head_shas` to load pipelines
       committers: [merge_request_diff: [:merge_request_diff_commits]],
-      suggested_reviewers: [:predictions]
+      suggested_reviewers: [:predictions],
+      diff_stats: [:latest_merge_request_diff]
     }
   end
 end

spec/requests/api/graphql/project/merge_requests_spec.rb

@@ -396,6 +396,18 @@ def execute_query
 
       include_examples 'N+1 query check', skip_cached: false
     end
+
+    context 'when requesting diffStats' do
+      let(:requested_fields) { ['diffStats { path }'] }
+
+      before do
+        create_list(:merge_request_diff, 2, merge_request: merge_request_a)
+        create_list(:merge_request_diff, 2, merge_request: merge_request_b)
+        create_list(:merge_request_diff, 2, merge_request: merge_request_c)
+      end
+
+      include_examples 'N+1 query check', skip_cached: false
+    end
   end
 
   describe 'performance' do