Don't fail in Diffusion if .gitmodules is missing

Summary:
See T1448. If this file isn't present, just move on instead of failing, since it's a (sort of) legitimate repository state.

Also fix some silliness a little later that got introduced in refactoring, I think.

Test Plan: Added an external to my test repo and removed ".gitmodules". Verified that the directory is now viewable after this patch.

Reviewers: btrahan, davidreuss, jungejason

Reviewed By: davidreuss

CC: aran

Maniphest Tasks: T1448

Differential Revision: https://secure.phabricator.com/D2922