completion: use __gitcomp_builtin in _git_pull
This is really nice. Since pull_options[] already declares all
passthru options to 'merge' or 'fetch', a single
git pull --git-completion-helper
would provide all completable options (--no- variants are a separate
issue). Dead shell variables can now be deleted.
New completable options are:
--allow-unrelated-histories
--ipv4
--ipv6
--jobs
--refmap=
--signoff
--strategy-option=
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>