checkout: Use fd-relative open of newly created directory

We were walking the full path again on our directories, no need to do
that.

src/libgsystem

@@ -1 +1 @@
-Subproject commit 31cd0b96bba5ccf391f4c4f5d94cddce44c65793
+Subproject commit 7a2499708b6aa0be5a67e4a75de66fb9c18516af

src/libostree/ostree-repo-checkout.c

@@ -568,8 +568,9 @@ checkout_tree_at (OstreeRepo                        *self,
         }
     }
 
-  if (!gs_file_open_dir_fd (destination, &destination_dfd,
+  if (!gs_file_open_dir_fd_at (destination_parent_fd, destination_name,
-                            cancellable, error))
+                               &destination_dfd,
+                               cancellable, error))
     goto out;
 
   /* Set the xattrs now, so any derived labeling works */