submodule--helper: use correct display path helper

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>