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.php

@@ -473,5 +473,14 @@ class IDF_Scm
     {
         return str_replace('%2F', '/', rawurlencode($path));
     }
+
+    /**
+     * Returns the number of slashes and preceeding path components
+     * that should be stripped from paths in the SCM's diff output
+     */
+    public function getDiffPathStripLevel()
+    {
+        return 0;
+    }
 }