Adding initial feature flag line

app/serializers/merge_request_diff_entity.rb

@@ -31,7 +31,11 @@ class MergeRequestDiffEntity < Grape::Entity
     next unless project
-    merge_request_version_path(project, merge_request, merge_request_diff)
+    if Feature.enabled?(:default_merge_ref_for_diffs)
+      diffs_project_merge_request_path(project, merge_request, diff_head: true)
+    else
+      merge_request_version_path(project, merge_request, merge_request_diff)
+    end
   end
   expose :head_version_path do |merge_request_diff|