Change "any" to explicitly list revision control systems

Summary: Using array('any') to represent array('git', 'hg', 'svn') is a bit magical and leads to a lot of special-casing.

Test Plan: Verified that tab completion (ala ArcanistShellCompleteWorkflow) still worked.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11604