Fix a fatal for git uncommitted changes in new arc projects

Summary:
If you run arc diff in a repository which:

• has uncommitted or untracked changes; and
• has a .arcconfig with a never-before-seen project ID;
• we fatal: http://pastebin.com/raw.php?i=ykpfr4MT

This patch is a bit iffy, open to alternatives. The "right" patch is probably an arcanistproject.query which behaves more sensibly.

I return array() directly since we'll later create the project.

Test Plan: Ran arc diff in a repository with untracked files or uncommitted changes and a new project ID.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4057