Rewrite the diff parser and reduce the memory footprint.

The diff parser code was rewritten for clarity and speed and now handles
a couple of ugly cornercases, like SVN's property change output and single
change chunks, much better. Since the path parsing was unified as well,
the SCM interface gained a new method `getPathStripLevel()` which determines
how many path components need to be shoven off a file name for the SCM
to form a valid path in the workspace (similar to patch(1)'s --strip option).
Fixes issue 627. Automated tests follow.

src/IDF/Scm/Mercurial.php

@@ -464,4 +464,12 @@ class IDF_Scm_Mercurial extends IDF_Scm
                        escapeshellarg($commit));
         return new Pluf_HTTP_Response_CommandPassThru($cmd, 'application/x-zip');
     }
+
+    /**
+     * @see IDF_Scm::getDiffPathStripLevel()
+     */
+    public function getDiffPathStripLevel()
+    {
+        return 1;
+    }
 }