avoid provenance update if new_value and old_value are equal

smash/web/importer/csv_tns_visit_import_reader.py

@@ -96,6 +96,8 @@ class TnsCsvVisitImportReader:
 
                         for field, new_value in changes:
                             old_value = getattr(visit, field)
+                            if old_value == new_value:
+                                continue
                             description = u'{} changed from "{}" to "{}"'.format(field, old_value, new_value)
                             p = Provenance(modified_table=Visit._meta.db_table,
                                         modified_table_id=visit.id,
@@ -143,6 +145,8 @@ class TnsCsvVisitImportReader:
 
                         for field, new_value in changes:
                             old_value = getattr(appointment, field)
+                            if old_value == new_value:
+                                continue
                             description = u'{} changed from "{}" to "{}"'.format(field, old_value, new_value)
                             p = Provenance(modified_table=Appointment._meta.db_table,
                                         modified_table_id=appointment.id,