Remove unnecessary db call when replacing.

When replacing a has_many association with the same one, there is no
need to do a round-trip to the db to create/and drop a new transaction.

[fixes #14220]

activerecord/lib/active_record/associations/collection_association.rb

@@ -359,7 +359,9 @@ def replace(other_array)
         if owner.new_record?
           replace_records(other_array, original_target)
         else
-          transaction { replace_records(other_array, original_target) }
+          if other_array != original_target
+            transaction { replace_records(other_array, original_target) }
+          end
         end
       end
 

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1242,6 +1242,16 @@ def test_replace_failure
     assert_equal orig_accounts, firm.accounts
   end
 
+  def test_replace_with_same_content
+    firm = Firm.first
+    firm.clients = []
+    firm.save
+
+    assert_queries(0, ignore_none: true) do
+      firm.clients = []
+    end
+  end
+
   def test_transactions_when_replacing_on_persisted
     good = Client.new(:name => "Good")
     bad  = Client.new(:name => "Bad", :raise_on_save => true)