[git] Override diff.submodule so git diff output is always parseable

Test Plan:
Removed a submodule with diff.submodule set to log, saw
arc diff error; with this change, it no longer does.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T10881

Differential Revision: https://secure.phabricator.com/D17327