[IMP] mail: use has_group to check a user is part of a group

`has_groups` is more efficient, and won't lead to useless
prefetches.

opw-634402

addons/mail/mail_thread.py

@@ -340,9 +340,7 @@ class mail_thread(osv.AbstractModel):
         options = {
             'display_log_button': False
         }
-        group_ids = self.pool.get('res.users').browse(cr, uid, uid, context=context).groups_id
-        group_user_id = self.pool.get("ir.model.data").get_object_reference(cr, uid, 'base', 'group_user')[1]
-        is_employee = group_user_id in [group.id for group in group_ids]
+        is_employee = self.pool['res.users'].has_group(cr, uid, 'base.group_user')
         if is_employee:
             options['display_log_button'] = True
         return options