Allow Diffusion to display the initial commit in Git repositories

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