[svn1.7] Fix thrown exception when browsing diffusion with added directories

Summary: svn cat returns a non-zero exit status when trying to cat a directory.

Test Plan: Browsed diffusion in my sandbox

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1882