parse ls-tree with git-submodule correctly

Summary:
git ls-tree gives type 'commit' for submodules.

This code-path is only used when a binary file is present in the diff.

Test Plan: arc diff with a binary file

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6943