Fix bug

Author: lunny

services/pull/update.go

@@ -39,11 +39,13 @@ func Update(ctx context.Context, pr *issues_model.PullRequest, doer *user_model.
 		return fmt.Errorf("unable to load BaseRepo for PR[%d] during update-by-merge: %w", pr.ID, err)
 	}
 
-	diffCount, err := GetDiverging(ctx, pr)
-	if err != nil {
-		return err
-	} else if diffCount.Behind == 0 {
-		return fmt.Errorf("HeadBranch of PR %d is up to date", pr.Index)
+	if pr.ID > 0 { // only a real PR needs to check this
+		diffCount, err := GetDiverging(ctx, pr)
+		if err != nil {
+			return err
+		} else if diffCount.Behind == 0 {
+			return fmt.Errorf("HeadBranch of PR %d is up to date", pr.Index)
+		}
 	}
 
 	if err := pr.LoadHeadRepo(ctx); err != nil {