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'

src/uu/cp/src/cp.rs

@@ -1084,7 +1084,7 @@ fn copy_directory(
         return Err(format!(
             "cannot copy a directory, {}, into itself, {}",
             root.quote(),
-            target.quote()
+            root.join(target).quote()
         )
         .into());
     }

tests/by-util/test_cp.rs

@@ -2077,3 +2077,13 @@ 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");
+    ucmd.args(&["-R", "d", "d"])
+        .fails()
+        .stderr_only("cp: cannot copy a directory, 'd', into itself, 'd/d'");
+}