fuse: Properly set uid/gid for directories

In #2584 this was changed to use the uid/gid of the root node. This
would be okay for the top-level directory of a snapshot, however, this
change also applied to normal directories within a snapshot. This
change reverts the problematic part and adds a test that directory
attributes are represented correctly.

internal/fuse/dir.go

@@ -111,8 +111,11 @@ func (d *dir) Attr(ctx context.Context, a *fuse.Attr) error {
 	debug.Log("Attr()")
 	a.Inode = d.inode
 	a.Mode = os.ModeDir | d.node.Mode
-	a.Uid = d.root.uid
-	a.Gid = d.root.gid
+
+	if !d.root.cfg.OwnerIsRoot {
+		a.Uid = d.node.UID
+		a.Gid = d.node.GID
+	}
 	a.Atime = d.node.AccessTime
 	a.Ctime = d.node.ChangeTime
 	a.Mtime = d.node.ModTime