fix tab complete for out-of-workdir

Summary:
see https://github.com/facebook/phabricator/issues/546 - arc complete blows up when not
in a workdir.

There's no "is ArcanistWorkingCopyIdentity object valid" method, but getVCSType() looks like the
closest match.

git grep for `getProjectRoot` didn't reveal any more problmatic call sites.

Test Plan: `arc [tab] [tab]`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8578