completion: use __gitcomp_builtin in _git_init

The new completable option is --separate-git-dir=.

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

@@ -1606,7 +1606,7 @@ _git_init ()
 		return
 		;;
 	--*)
-		__gitcomp "--quiet --bare --template= --shared --shared="
+		__gitcomp_builtin init
 		return
 		;;
 	esac