cp: correct error message on copying dir to itself

Correct the error message produced when attempting to copy a directory
into itself with `cp`. Before this commit, the error message was

    $ cp -R d d
    cp: cannot copy a directory, 'd', into itself, 'd'

After this commit, the error message is

    $ cp -R d d
    cp: cannot copy a directory, 'd', into itself, 'd/d'

tests/by-util/test_cp.rs

@@ -2095,3 +2095,31 @@ fn test_cp_mode_hardlink_no_dereference() {
     assert!(at.symlink_exists("z"));
     assert_eq!(at.read_symlink("z"), "slink");
 }
+
+/// Test that copying a directory to itself is disallowed.
+#[test]
+fn test_copy_directory_to_itself_disallowed() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.mkdir("d");
+    #[cfg(not(windows))]
+    let expected = "cp: cannot copy a directory, 'd', into itself, 'd/d'";
+    #[cfg(windows)]
+    let expected = "cp: cannot copy a directory, 'd', into itself, 'd\\d'";
+    ucmd.args(&["-R", "d", "d"]).fails().stderr_only(expected);
+}
+
+/// Test that copying a nested directory to itself is disallowed.
+#[test]
+fn test_copy_nested_directory_to_itself_disallowed() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.mkdir("a");
+    at.mkdir("a/b");
+    at.mkdir("a/b/c");
+    #[cfg(not(windows))]
+    let expected = "cp: cannot copy a directory, 'a/b', into itself, 'a/b/c/b'";
+    #[cfg(windows)]
+    let expected = "cp: cannot copy a directory, 'a/b', into itself, 'a/b/c\\b'";
+    ucmd.args(&["-R", "a/b", "a/b/c"])
+        .fails()
+        .stderr_only(expected);
+}