rust: simplify overly complicated expression

Differential Revision: https://phab.mercurial-scm.org/D6592

rust/hg-cpython/src/dirstate.rs

@@ -291,8 +291,7 b' py_class!(pub class Dirs |py| {'
         Ok(self
             .dirs_map(py)
             .borrow()
-            .get(&item.extract::<PyBytes>(py)?.data(py).to_owned())
-            .is_some())
+            .contains_key(item.extract::<PyBytes>(py)?.data(py).as_ref()))
     }
 });