Do not update multivalue record if the value is not modified

When a ValueMixin is used for many MultiValue fields, they are created with
default values when the first MultiValue is set.
But often the other MultiValue fields are set with the default value so by not
forcing the change, we avoid a unnecessary UPDATE query.

trytond/trytond/model/multivalue.py

@@ -84,7 +84,14 @@ class MultiValueMixin(object):
         if not values:
             values = [self.multivalue_record(name, **pattern)]
         for record in values:
-            setattr(record, name, value)
+            current_value = getattr(record, name, None)
+            if isinstance(current_value, Model):
+                current_value = current_value.id
+            elif isinstance(current_value, (list, tuple)):
+                current_value = [
+                    r.id if isinstance(r, Model) else r for r in current_value]
+            if current_value != value:
+                setattr(record, name, value)
         if save:
             Value.save(values)
         else: