Added treeclosed PR url in message

Author: Rhythm1710

src/bors/handlers/review.rs

@@ -145,8 +145,11 @@ pub(super) async fn command_close_tree(
         repo_state.repository(),
         TreeState::Closed {
             priority,
-            // TODO: figure out a HTML URL to the PR
-            source: "TODO".to_string(),
+            source: format!(
+                "https://github.com/{}/pull/{}",
+                repo_state.repository(),
+                pr.number
+            ),
         },
     )
     .await?;