Handle SVN root changes correctly

Summary: If there is a change in SVN root (perhaps properties change) then we try to list its parent (which doesn't exist) and mark the root itself as deleted.

Test Plan: Parsed SVN commit with property change of root.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5709