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