completion: teach archive to use __gitcomp_builtin

Currently, _git_archive() uses a hardcoded list of options for its completion. However, we can use __gitcomp_builtin() to get a dynamically generated list of completions instead. Teach _git_archive() to use __gitcomp_builtin() so that newly implemented options in archive will be automatically completed without any mucking around in git-completion.bash. While we're at it, teach it to complete the missing `--worktree-attributes` option as well. Unfortunately, since some args are passed through from cmd_archive() to write_archive() (which calls parse_archive_args()), there's no way that a `--git-completion-helper` arg can end up reaching parse_archive_args() since the first call to parse_options() will end up calling exit(0). As a result, we have to carry the options supported by write_archive() in the hardcoded string. Signed-off-by: Denton Liu <liu.denton@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>