Keep repo-related path handling in one place. This will make it easier
to add submodule/multiworktree support later.
This automatically adds the "if submodule then use the submodule version
of git_path" to other call sites too. But it does not mean those
operations are submodule-ready. Not yet.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>