date: show error when reading dirs for -f arg (#4572)

Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>

tests/by-util/test_date.rs

@@ -283,6 +283,16 @@ fn test_date_for_invalid_file() {
     );
 }
 
+#[test]
+fn test_date_for_dir_as_file() {
+    let result = new_ucmd!().arg("--file").arg("/").fails();
+    result.no_stdout();
+    assert_eq!(
+        result.stderr_str().trim(),
+        "date: expected file, got directory '/'",
+    );
+}
+
 #[test]
 fn test_date_for_file() {
     let (at, mut ucmd) = at_and_ucmd!();