[svn1.7] treat nonpositive rev numbers as added files

Summary:
svn 1.7 reports a rev number of -1 for added files; previous versions
reported a rev number of 0. Interpret nonpositive rev numbers to mean
the file was added.

Test Plan: ran arc diff on an svn1.7 working copy with added files

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1867

src/workflow/diff/ArcanistDiffWorkflow.php

@@ -652,7 +652,7 @@ EOTEXT
 
       // Remove all files with baserev "0"; these files are new.
       foreach ($bases as $path => $baserev) {
-        if ($bases[$path] == 0) {
+        if ($bases[$path] <= 0) {
           unset($bases[$path]);
         }
       }