rust-clippy: fix most warnings in `hg-core` All of these are simple changes that for the most part are clear improvements and the rest are at most equivalent. The remaining warnings have to be fixed either with a bigger refactor like for the nested "revlog" module, or in the dependency `bytes-cast`, which we own. This will be done sometime in the future.

--- a/rust/hg-core/src/dirstate.rs	Mon Jan 09 19:14:14 2023 +0100
+++ b/rust/hg-core/src/dirstate.rs	Mon Jan 09 19:18:43 2023 +0100
@@ -32,7 +32,7 @@
     };
 
     pub fn is_merge(&self) -> bool {
-        return !(self.p2 == NULL_NODE);
+        !(self.p2 == NULL_NODE)
     }
 }