Summary: See T507. Since you can't do "xxxxxxxx^" where "xxxxxxxx" is the first commit in a repository, fall back to diffing against the empty tree if we fail to diff against the parent commit. Test Plan: Looked at the first commit in libphutil on my local. Reviewers: edward, jungejason, nh, tuomaspelkonen, aran Reviewed By: nh CC: aran, edward, epriestley, nh Differential Revision: 953