Invalidate SVN status cache after adding files

Summary: Cache invalidation is really one of the two hardest computer science problems.

Test Plan:
  lang=sh
  # in Subversion repository
  $ arc diff .arcconfig # untracked

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Differential Revision: https://secure.phabricator.com/D5600