rust-clippy: add `is_empty` method to please the `clippy` gods

I can see it being useful anyway.

rust/hg-core/src/dirstate_tree/dirstate_map.rs

@@ -1380,6 +1380,10 b' impl OwningDirstateMap {'
         map.nodes_with_entry_count as usize
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     pub fn contains_key(
         &self,
         key: &HgPath,