rust: do a clippy pass This is the result of running `cargo clippy` on hg-core/hg-cpython and fixing the lints that do not require too much code churn (and would warrant a separate commit/complete refactor) and only come from our code (a lot of warnings in hg-cpython come from `rust-cpython`). Most of those were good lints, two of them was the linter not being smart enough (or compiler to get up to `clippy`'s level depending on how you see it). Maybe in the future we could have `clippy` be part of the CI. Differential Revision: https://phab.mercurial-scm.org/D8635

--- a/rust/hg-cpython/src/dirstate/status.rs	Mon Jun 15 15:14:16 2020 -0400
+++ b/rust/hg-cpython/src/dirstate/status.rs	Mon Jun 15 18:26:40 2020 +0200
@@ -236,12 +236,10 @@
 
             build_response(py, lookup, status_res, all_warnings)
         }
-        e => {
-            return Err(PyErr::new::<ValueError, _>(
-                py,
-                format!("Unsupported matcher {}", e),
-            ));
-        }
+        e => Err(PyErr::new::<ValueError, _>(
+            py,
+            format!("Unsupported matcher {}", e),
+        )),
     }
 }