Make the type optional when parsing chunk lines.

git version 1.7.11.3 is generating a ```git diff``` where unchanged blank lines have no ```" "``` prefix.

Author: Adrian Clay

src/Parser.php

@@ -110,7 +110,7 @@ private function parseFileDiff(Diff $diff, array $lines)
             $last      = null;
 
             while (count($lines) &&
-                  (preg_match('(^(?P<type>[+ -])(?P<line>.*))', $last = array_shift($lines), $match) ||
+                  (preg_match('(^(?P<type>[+ -])?(?P<line>.*))', $last = array_shift($lines), $match) ||
                   (strpos($last, '\\ No newline at end of file' ) === 0))) {
                 if (count($match)) {
                     $type = Line::UNCHANGED;