Add #all_includes method to reflections

`yield` instead of relying on checking if the reflection is equal to the
`chain_head`.

activerecord/lib/active_record/associations/association_scope.rb

@@ -121,6 +121,8 @@ def initialize(reflection, alias_name)
           super(reflection)
           @alias_name = alias_name
         end
+
+        def all_includes; nil; end
       end
 
       def get_chain(refl, association, tracker)
@@ -161,7 +163,7 @@ def add_constraints(scope, owner, assoc_klass, refl, connection, chain_head, cha
               scope.merge! item.except(:where, :includes, :bind)
             end
 
-            if reflection == chain_head
+            reflection.all_includes do
               scope.includes! item.includes_values
             end
 

activerecord/lib/active_record/reflection.rb

@@ -970,6 +970,8 @@ def alias_candidate(name)
       def alias_name
         Arel::Table.new(table_name)
       end
+
+      def all_includes; yield; end
     end
   end
 end