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

src/applications/differential/parser/changeset/DifferentialChangesetParser.php

@@ -1544,6 +1544,7 @@ class DifferentialChangesetParser {
       DifferentialChangeType::FILE_DIRECTORY  => 'directory',
       DifferentialChangeType::FILE_BINARY     => 'binary file',
       DifferentialChangeType::FILE_SYMLINK    => 'symlink',
+      DifferentialChangeType::FILE_SUBMODULE  => 'submodule',
     );
 
     static $changes = array(