Add Singleton in NullMutationTracker class

to reduce allocation of same object

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -133,7 +133,7 @@ module ActiveRecord
       end
 
       def previous_mutation_tracker
-        @previous_mutation_tracker ||= NullMutationTracker.new
+        @previous_mutation_tracker ||= NullMutationTracker.instance
       end
 
       def cache_changed_attributes

activerecord/lib/active_record/attribute_mutation_tracker.rb

@@ -46,6 +46,8 @@ module ActiveRecord
   end
 
   class NullMutationTracker # :nodoc:
+    include Singleton
+
     def changed_values
       {}
     end