refactor(tvix/store): impl Default for MemoryDirectoryService

Change-Id: Ie75a1e67c8c0499ff10f60d90251b768937dad3a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8235
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/store/src/tests/utils.rs

@@ -14,7 +14,7 @@ pub fn gen_chunk_service() -> impl ChunkService + Clone {
 }
 
 pub fn gen_directory_service() -> impl DirectoryService + Send + Sync + Clone + 'static {
-    MemoryDirectoryService::new()
+    MemoryDirectoryService::default()
 }
 
 pub fn gen_pathinfo_service() -> impl PathInfoService {