Avoid emit [behind-upstream] more than once

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

src/handlers/check_commits/behind_upstream.rs

@@ -27,8 +27,9 @@ pub(super) async fn behind_upstream(
             days_old
         );
 
+        // The message remain the same to ensure the warning is emit only once.
         Some(format!(
-            r"This PR is based on an [upstream commit]({upstream_commit_url}) that is {days_old} days old.
+            r"This PR is based on an [upstream commit]({upstream_commit_url}) that is older than {age_threshold} days.
 
 *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).*",
         ))