radicle: add notification count grouped by repo method — heartwood

While the total notification count and the notification count by repo are needed, for most UIs users will want to see the total notification count grouped by repo. This new method will provide this without the need of checking all possible repos to see if they have notifications.

         Ok(count)
     }
     /// Get the notification for the given repo.
     /// Get the total notification count by repos.
     pub fn count_by_repos(
         &self,
     ) -> Result<impl Iterator<Item = Result<(RepoId, usize), Error>> + '_, Error> {
         let stmt = self.db.prepare(
             "SELECT repo, COUNT(*) as count
             FROM `repository-notifications`
             GROUP BY repo",
         )?;
         Ok(stmt.into_iter().map(|row| {
             let row = row?;
             let count = row.try_read::<i64, _>("count")? as usize;
             let repo = row.try_read::<RepoId, _>("repo")?;
             Ok((repo, count))
         }))
     }
     /// Get the notification count for the given repo.
     pub fn count_by_repo(&self, repo: &RepoId) -> Result<usize, Error> {
         let mut stmt = self
             .db

     }
     #[test]
     fn test_count_by_repos() {
         let mut db = Store::open(":memory:").unwrap();
         let repo1 = arbitrary::gen::<RepoId>(1);
         let repo2 = arbitrary::gen::<RepoId>(1);
         let oid = arbitrary::oid();
         let time = LocalTime::from_millis(32188142);
         let update1 = RefUpdate::Created {
             name: refname!("refs/heads/feature/1"),
             oid,
         };
         let update2 = RefUpdate::Created {
             name: refname!("refs/heads/feature/2"),
             oid,
         };
         let update3 = RefUpdate::Created {
             name: refname!("refs/heads/feature/3"),
             oid,
         };
         assert!(db.insert(&repo1, &update1, time).unwrap());
         assert!(db.insert(&repo1, &update2, time).unwrap());
         assert!(db.insert(&repo2, &update3, time).unwrap());
         let mut count = db.count_by_repos().unwrap();
         assert_eq!(count.next().unwrap().unwrap(), (repo1, 2));
         assert_eq!(count.next().unwrap().unwrap(), (repo2, 1));
     }
     #[test]
     fn test_branch_notifications() {
         let repo = arbitrary::gen::<RepoId>(1);
         let old = arbitrary::oid();