completion: use __gitcomp_builtin in _git_config

The new completable options are:

--blob=
--bool
--bool-or-int
--edit
--expiry-date
--get-color
--get-colorbool
--get-urlmatch
--includes
--int
--null
--path
--show-origin

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