basename: implement and test repeated flags and arguments

Note in particular that `args_override_self` would *NOT* work here,
since it would in all cases cause `options::NAME` to override itself,
or interfere with `trailing_var_arg`.

tests/by-util/test_basename.rs

@@ -202,6 +202,54 @@ fn test_invalid_arg() {
     new_ucmd!().arg("--definitely-invalid").fails().code_is(1);
 }
 
+#[test]
+fn test_repeated_multiple() {
+    new_ucmd!()
+        .args(&["-aa", "-a", "foo"])
+        .succeeds()
+        .stdout_is("foo\n");
+}
+
+#[test]
+fn test_repeated_multiple_many() {
+    new_ucmd!()
+        .args(&["-aa", "-a", "1/foo", "q/bar", "x/y/baz"])
+        .succeeds()
+        .stdout_is("foo\nbar\nbaz\n");
+}
+
+#[test]
+fn test_repeated_suffix_last() {
+    new_ucmd!()
+        .args(&["-s", ".h", "-s", ".c", "foo.c"])
+        .succeeds()
+        .stdout_is("foo\n");
+}
+
+#[test]
+fn test_repeated_suffix_not_first() {
+    new_ucmd!()
+        .args(&["-s", ".h", "-s", ".c", "foo.h"])
+        .succeeds()
+        .stdout_is("foo.h\n");
+}
+
+#[test]
+fn test_repeated_suffix_multiple() {
+    new_ucmd!()
+        .args(&["-as", ".h", "-a", "-s", ".c", "foo.c", "bar.c", "bar.h"])
+        .succeeds()
+        .stdout_is("foo\nbar\nbar.h\n");
+}
+
+#[test]
+fn test_repeated_zero() {
+    new_ucmd!()
+        .args(&["-zz", "-z", "foo/bar"])
+        .succeeds()
+        .stdout_is("bar\0");
+}
+
 #[test]
 fn test_zero_does_not_imply_multiple() {
     new_ucmd!()