modify gc layout

examples/tunasync.conf

@@ -13,7 +13,8 @@ max_retry = 2
 [btrfs]
 service_dir = "{mirror_root}/_current/{mirror_name}"
 working_dir = "{mirror_root}/_working/{mirror_name}"
-gc_dir = "{mirror_root}/_gc/{mirror_name}_{{timestamp}}"
+gc_root = "{mirror_root}/_garbage/"
+gc_dir = "{mirror_root}/_garbage/_gc_{mirror_name}_{{timestamp}}"
 
 [[mirrors]]
 name = "archlinux"