completion: use __gitcomp_builtin in _git_am

The new completable options are:

--directory
--exclude
--gpg-sign
--include
--keep-cr
--keep-non-patch
--message-id
--no-keep-cr
--patch-format
--quiet
--reject
--resolvemsg=

In-progress options like --continue will be part of --git-completion-helper
then filtered out by _git_am() 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>