Upgrade rust/crates/structopt to 0.3.20

Test: make
Change-Id: I50a53b96c652aea7c84a572a87578cf3828c00e2
diff --git a/tests/regressions.rs b/tests/regressions.rs
new file mode 100755
index 0000000..876e7ed
--- /dev/null
+++ b/tests/regressions.rs
@@ -0,0 +1,25 @@
+use structopt::StructOpt;
+
+mod utils;
+use utils::*;
+
+#[test]
+fn invisible_group_issue_439() {
+    macro_rules! m {
+        ($bool:ty) => {
+            #[derive(Debug, StructOpt)]
+            struct Opts {
+                #[structopt(long = "x")]
+                x: $bool,
+            }
+        };
+    }
+
+    m!(bool);
+
+    let help = get_long_help::<Opts>();
+
+    assert!(help.contains("--x"));
+    assert!(!help.contains("--x <x>"));
+    Opts::from_iter_safe(&["test", "--x"]).unwrap();
+}