Fix getting changed files in SVN

Summary:
This is pretty awkward but I don't have anything better.

Also don't compute cache key if caching is disabled.

Test Plan:
  $ svn diff --xml --summarize '' -r 701319:HEAD
  $ svn diff --xml --summarize svn+ssh://tubbs/svnroot/projects/lolbunny -r 701319:HEAD

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4771