basename: handle paths comprising only slashes

Fix an issue where `basename` would print the empty path when provided
an input comprising only slashes (for example, "/" or "//" or
"///"). Now it correctly prints the path "/".

tests/by-util/test_basename.rs

@@ -147,3 +147,30 @@ fn invalid_utf8_args_unix() {
     let os_str = OsStr::from_bytes(&source[..]);
     test_invalid_utf8_args(os_str);
 }
+
+#[test]
+fn test_root() {
+    new_ucmd!().arg("/").succeeds().stdout_is("/\n");
+}
+
+#[test]
+fn test_double_slash() {
+    // TODO The GNU tests seem to suggest that some systems treat "//"
+    // as the same directory as "/" directory but not all systems. We
+    // should extend this test to account for that possibility.
+    let expected = if cfg!(windows) { "\\" } else { "/\n" };
+    new_ucmd!().arg("//").succeeds().stdout_is(expected);
+    new_ucmd!()
+        .args(&["//", "/"])
+        .succeeds()
+        .stdout_is(expected);
+    new_ucmd!()
+        .args(&["//", "//"])
+        .succeeds()
+        .stdout_is(expected);
+}
+
+#[test]
+fn test_triple_slash() {
+    new_ucmd!().arg("///").succeeds().stdout_is("/\n");
+}