[IMP] runbot: add a message when a build error is merged

When digging into deactivated build errors, one cannot easily find why
an error was deactivated and to which one it was merged.

With this commit, a message is added in the chatter to explain where it
is merged.

runbot/models/build_error.py

@@ -199,6 +199,7 @@ class BuildError(models.Model):
                     error.team_id = previous_error.team_id
             previous_error.error_content_ids.write({'error_id': self})
             if not previous_error.test_tags:
+                previous_error.message_post(body=Markup('Error merged into %s') % error._get_form_link())
                 previous_error.active = False
 
     @api.model