completion: use __gitcomp_builtin in _git_cherry_pick

completion: use __gitcomp_builtin in _git_cherry_pick

The new completable options are:

--allow-empty
--allow-empty-message
--ff
--gpg-sign
--keep-redundant-commits
--strategy-option

In-progress options like --continue will be part of --git-completion-helper
then filtered out by _git_cherry_pick() unless the operation is in
progress. This helps keep marking of these operations in just one place.

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