watchman: use single-threaded async runtime

The `#[tokio::main]` annotation uses a multi-threaded runtime by
default. We don't need that for querying watchman. Switching to the
single-threaded runtime saves about 20 ms.

lib/src/working_copy.rs

@@ -623,7 +623,7 @@ impl TreeState {
     }
 
     #[cfg(feature = "watchman")]
-    #[tokio::main]
+    #[tokio::main(flavor = "current_thread")]
     #[instrument(skip(self))]
     pub async fn query_watchman(
         &self,