Short-circuit block-on if same thread (#862)

* Short-circuit `block_on` if same thread

* Avoid acquiring the lock

Author: MichaReiser

src/runtime.rs

@@ -172,8 +172,13 @@ impl Runtime {
         other_id: ThreadId,
         query_mutex_guard: QueryMutexGuard,
     ) -> BlockResult {
-        let dg = self.dependency_graph.lock();
         let thread_id = thread::current().id();
+        // Cycle in the same thread.
+        if thread_id == other_id {
+            return BlockResult::Cycle;
+        }
+
+        let dg = self.dependency_graph.lock();
 
         if dg.depends_on(other_id, thread_id) {
             return BlockResult::Cycle;