rust-nodemap: implement `PartialEq` without allocation

This was caught by `clippy`. It's probable that this ends up optimized out
by the compiler, but let's not rely on that.

rust/hg-core/src/revlog/node.rs

@@ -314,7 +314,7 impl From<Node> for NodePrefix {
 
 impl PartialEq<Node> for NodePrefix {
     fn eq(&self, other: &Node) -> bool {
-        Self::from(*other) == *self
+        self.data == other.data && self.nybbles_len() == other.nybbles_len()
     }
 }