Merge pull request #1336 from rinconjc/1321

Fix #1321 tsort handle self-loops

tests/test_tsort.rs

@@ -8,3 +8,10 @@ fn test_sort_call_graph() {
         .run()
         .stdout_is_fixture("call_graph.expected");
 }
+
+#[test]
+fn test_sort_self_loop() {
+    new_ucmd!()
+        .pipe_in("first first\nfirst second second second")
+        .succeeds().stdout_only("first\nsecond\n");
+}