Add upstream merge base commit link for [behind-upstream]

src/github.rs

@@ -2847,6 +2847,7 @@ pub struct GithubCommit {
     pub sha: String,
     pub commit: GithubCommitCommitField,
     pub parents: Vec<Parent>,
+    pub html_url: String,
 }
 
 #[derive(Debug, serde::Deserialize)]

src/handlers/check_commits.rs

@@ -246,6 +246,7 @@ fn dummy_commit_from_body(sha: &str, body: &str) -> GithubCommit {
             },
         },
         parents: vec![],
+        html_url: "".to_owned(),
     }
 }
 

src/handlers/check_commits/behind_upstream.rs

@@ -17,6 +17,8 @@ pub(super) async fn behind_upstream(
     let now = chrono::Utc::now().with_timezone(&commit_date.timezone());
     let days_old = (now - commit_date).num_days() as usize;
 
+    let upstream_commit_url = &compare.merge_base_commit.html_url;
+
     // First try the parent commit age check as it's more accurate
     if days_old > age_threshold {
         log::info!(
@@ -26,10 +28,9 @@ pub(super) async fn behind_upstream(
         );
 
         Some(format!(
-            r"This PR is based on an upstream commit that is {} days old.
+            r"This PR is based on an [upstream commit]({upstream_commit_url}) that is {days_old} days old.
 
 *It's recommended to update your branch according to the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/contributing.html#keeping-your-branch-up-to-date).*",
-            days_old
         ))
     } else {
         // Parent commit is not too old, log and do nothing