[1.6.x] Prevented arbitrary file inclusion with {% ssi %} tag and relative paths.

Thanks Rainer Koirikivi for the report and draft patch.

This is a security fix; disclosure to follow shortly.

Backport of 7fe5b656c9 from master

django/template/defaulttags.py

@@ -1,6 +1,7 @@
 """Default tags used by the template system, available to all templates."""
 from __future__ import unicode_literals
 
+import os
 import sys
 import re
 from datetime import datetime
@@ -332,6 +333,7 @@ class RegroupNode(Node):
         return ''
 
 def include_is_allowed(filepath):
+    filepath = os.path.abspath(filepath)
     for root in settings.ALLOWED_INCLUDE_ROOTS:
         if filepath.startswith(root):
             return True