[PORT] Fix code owners will not be mentioned when a pull request comes from a forked repository (gitea#30476)

Fix #30277
Caused by #29783

---

- Resolves #5842
- Regression from #2855

(cherry picked from commit c63060b130d34e3f03f28f4dccbf04d381a95c17)

services/issue/pull.go

@@ -51,14 +51,14 @@ func PullRequestCodeOwnersReview(ctx context.Context, issue *issues_model.Issue,
 		return nil, err
 	}
 
-	if pr.HeadRepo.IsFork {
-		return nil, nil
-	}
-
 	if err := pr.LoadBaseRepo(ctx); err != nil {
 		return nil, err
 	}
 
+	if pr.BaseRepo.IsFork {
+		return nil, nil
+	}
+
 	repo, err := gitrepo.OpenRepository(ctx, pr.BaseRepo)
 	if err != nil {
 		return nil, err