rust-clippy: use `write_all` since we're not expecting a partial write

rust-clippy: use `write_all` since we're not expecting a partial write

Clippy caught this. This is a good lint in general, but here this was not
really a bug in this example code.

--- a/rust/hg-core/examples/nodemap/main.rs	Mon Jan 09 18:09:26 2023 +0100
+++ b/rust/hg-core/examples/nodemap/main.rs	Mon Jan 09 18:15:29 2023 +0100
@@ -40,7 +40,7 @@
         nm.insert(index, index.node(rev).unwrap(), rev).unwrap();
     }
     eprintln!("Nodemap constructed in RAM in {:?}", start.elapsed());
-    file.write(&nm.into_readonly_and_added_bytes().1)?;
+    file.write_all(&nm.into_readonly_and_added_bytes().1)?;
     eprintln!("Nodemap written to disk");
     Ok(())
 }