R1:ffd46df597c6

Avoid error in blaming empty file

Summary: Fixes T2389, resolves TODO.

Test Plan: Blamed seemingly binary file.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2389

Differential Revision: https://secure.phabricator.com/D4604