copies-rust: remove an unnecessary format!() inside assert!()

The `assert!()` macro supports formatting. Rust 2021 no longer
supports an unnecessary `format!()` inside it. I noticed because I
recently update my Rust toolchain and `test-check-cargo-lock.t`
started failing.

Differential Revision: https://phab.mercurial-scm.org/D10085

rust/hg-core/src/copy_tracing.rs

@@ -357,7 +357,7 b' impl TwoWayPathMap {'
     }
 
     fn untokenize(&self, token: PathToken) -> &HgPathBuf {
-        assert!(token < self.path.len(), format!("Unknown token: {}", token));
+        assert!(token < self.path.len(), "Unknown token: {}", token);
         &self.path[token]
     }
 }