Fix parsing of Git branches with common and useful name "0"
Summary: See https://discourse.phabricator-community.org/t/arc-has-a-spurious-failure-on-arc-diff/521. Oh, PHP!
Test Plan: Created a branch named "0", ran arc diff. Before: fatal. After: this beautiful revision.
Reviewers: amckinley
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18678