Use preload to avoid Rails using JOIN

Fixes https://sentry.gitlap.com/gitlab/gitlabcom/issues/26786/

app/controllers/projects/merge_requests_controller.rb

@@ -41,7 +41,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @collection_type    = "MergeRequest"
     @merge_requests     = merge_requests_collection
     @merge_requests     = @merge_requests.page(params[:page])
-    @merge_requests     = @merge_requests.includes(merge_request_diff: :merge_request)
+    @merge_requests     = @merge_requests.preload(merge_request_diff: :merge_request)
     @issuable_meta_data = issuable_meta_data(@merge_requests, @collection_type)
 
     if @merge_requests.out_of_range? && @merge_requests.total_pages != 0