submodule--helper: use correct display path helper

Replace a chunk of code in update_submodule() with an equivalent do_get_submodule_displaypath() invocation. This is already tested by t/t7406-submodule-update.sh:'submodule update --init --recursive from subdirectory', so no tests are added. The two are equivalent because: - Exactly one of recursive_prefix|prefix is non-NULL at a time; prefix is set at the superproject level, and recursive_prefix is set when recursing into submodules. There is also a BUG() statement in get_submodule_displaypath() that asserts that both cannot be non-NULL. - In get_submodule_displaypath(), get_super_prefix() always returns NULL because "--super-prefix" is never passed. Thus calling it is equivalent to calling do_get_submodule_displaypath() with super_prefix = NULL. Therefore: - When recursive_prefix is non-NULL, prefix is NULL, and thus get_submodule_displaypath() just returns prefixed_path. This is identical to calling do_get_submodule_displaypath() with super_prefix = recursive_prefix because the return value is still the concatenation of recursive_prefix + update_data->sm_path. - When prefix is non-NULL, prefixed_path = update_data->sm_path. Thus calling get_submodule_displaypath() with prefixed_path is equivalent to calling do_get_submodule_displaypath() with update_data->sm_path Signed-off-by: Glen Choo <chooglen@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>