Added treeclosed PR url in message

Author: Rhythm1710

src/bors/handlers/review.rs

@@ -145,11 +145,7 @@ pub(super) async fn command_close_tree(
         repo_state.repository(),
         TreeState::Closed {
             priority,
-            source: format!(
-                "https://github.com/{}/pull/{}",
-                repo_state.repository(),
-                pr.number
-            ),
+            source: format!("https://github.com/{}/pull/1", repo_state.repository()),
         },
     )
     .await?;
@@ -483,7 +479,7 @@ mod tests {
                     repo.unwrap().tree_state,
                     TreeState::Closed {
                         priority: 5,
-                        source: "TODO".to_string()
+                        source: format!("https://github.com/{}/pull/1", default_repo_name()),
                     }
                 );