Fix snapdiff arg parsing.
diff --git a/scripts/snapdiff.py b/scripts/snapdiff.py
index 4eb60a0..fda5570 100755
--- a/scripts/snapdiff.py
+++ b/scripts/snapdiff.py
@@ -83,8 +83,8 @@
     if len(args) != 2:
         optparser.error('incorrect number of arguments')
 
-    ref_prefix = sys.argv[1]
-    src_prefix = sys.argv[2]
+    ref_prefix = args[0]
+    src_prefix = args[1]
 
     if options.output:
         html = open(options.output, 'wt')