Merge branch 'strong-memoize-build-merge-request' into 'master'

Use StrongMemoize for Ci::Build#merge_request

See merge request gitlab-org/gitlab!27577

app/models/ci/build.rb

@@ -598,13 +598,9 @@ module Ci
     end
 
     def merge_request
-      return @merge_request if defined?(@merge_request)
-
-      @merge_request ||=
-        begin
+      strong_memoize(:merge_request) do
         merge_requests = MergeRequest.includes(:latest_merge_request_diff)
-            .where(source_branch: ref,
-                   source_project: pipeline.project)
+          .where(source_branch: ref, source_project: pipeline.project)
           .reorder(iid: :desc)
 
         merge_requests.find do |merge_request|