Add tests for git submodules, based on current behavior
This adds tests that detail the current behavior of arc in
the presence of git submodules.
Test Plan: No behavior change; wrote the tests such that they pass.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D18841