Fixed bug when looking at a commit which is a merge.

When you merge you do not get always a diff of files. This fix correct
that for the case of commit 9a2b8e249a.

src/IDF/Diff.php

@@ -46,6 +46,7 @@ class IDF_Diff
         $current_chunk = 0;
         $lline = 0;
         $rline = 0;
+        $files = array();
         foreach ($this->lines as $line) {
             if (0 === strpos($line, 'diff --git a')) {
                 $current_file = self::getFile($line);