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>

Author: pclouds

contrib/completion/git-completion.bash

@@ -2252,14 +2252,7 @@ _git_config ()
 	esac
 	case "$cur" in
 	--*)
-		__gitcomp "
-			--system --global --local --file=
-			--list --replace-all
-			--get --get-all --get-regexp
-			--add --unset --unset-all
-			--remove-section --rename-section
-			--name-only
-			"
+		__gitcomp_builtin config
 		return
 		;;
 	branch.*.*)