Free the root tree object buffer when we're done, plugging a minor leak
in generate_tar(). Note: we cannot simply free(tree.buf) because this
pointer is modified by tree_entry() calls in traverse_tree().
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <junkio@cox.net>