Support short aliases and repeatable arguments in Arcanist Workflow arguments

Summary: Ref T13546. Add support for short "-x" flags and repeatable "--say moo --say quack" flags.

Test Plan: In future changes, used "arc diff -m" and "arc land --onto ... --onto ...".

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21310