Make the Diffusion warning about "svnlook" and PATH more clear
See https://discourse.phabricator-community.org/t/display-error-on-the-status-page-for-svn-repos/2443 for discussion.
The UI currently shows a misleading warning that looks like "found svnlook; can't find svnlook".
It actually means "found svnlook, but when Subversion wipes PATH before executing commit hooks, we will no longer be able to find it".
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20210