Improve Diffusion parsing of submoudule changes

Summary: We currently parse these as directory changes and discard them. Instead, parse them as a new "SUBMODULE" type of change.

Test Plan:

• Reparsed a commit which changes submodules and verified it parses correctly.
• Reparsed a commit which adds submodules and verified it parses correctly.

Reviewers: btrahan, kdeggelman

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1815