Function change

backend/src/git/activity.rs:1534–1548

(topic: &RepoTopic, previous: Option<Timerange>)

        }

        fn change(topic: &RepoTopic, previous: Option<Timerange>) -> Change {
            let mut parent_topics = BTreeSet::new();
            for parent in &topic.parent_topics {
                parent_topics.insert(parent.id.to_owned());
            }

            Change::RemoveTopicTimerange(RemoveTopicTimerange {
                actor_id: "2".to_owned(),
                date: chrono::Utc::now(),
                id: Change::new_id(),
                parent_topics: parent_topics.iter().map(|path| path.to_owned()).collect(),
                previous_timerange: previous,
                updated_topic: TopicInfo::from(topic),
            })
        }

        #[test]
        fn simple_case() {