refs: rename get_ref_store() to get_submodule_ref_store() and make it public

This function is intended to replace *_submodule() refs API. It provides
a ref store for a specific submodule, which can be operated on by a new
set of refs API.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>