New completable options:

--deepen=
--ipv4
--ipv6
--jobs=
--multiple
--progress
--refmap=
--shallow-exclude=
--shallow-since=
--update-head-ok

Since _git_pull() needs fetch options too, $__git_fetch_options
remains. This variable will soon be gone after _git_pull() is updated.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>