Make "arc <workflow> --help" work again for workflows which haven't updated yet

Summary:
See https://discourse.phabricator-community.org/t/help-is-no-longer-present-for-arc-subcommands-in-todays-stable/3786.

The "--help" flag ends up falling through to the old "arcanist.php", where it becomes lost. Catch it earlier so "arc diff --help" prints diff help, for instance.

Test Plan: Ran arc help diff, arc diff --help, arc --help diff, and similar commands for updated workflows; got help.

Differential Revision: https://secure.phabricator.com/D21168