parseindex: also forward keyword argument in a debug wrapper

Otherwise, it gets in the way of a coming refactoring.

tests/test-parseindex.t

@@ -43,8 +43,8 b' We approximate that by reducing the read'
   > 
   > def opener(*args):
   >     o = vfs.vfs(*args)
-  >     def wrapper(*a):
-  >         f = o(*a)
+  >     def wrapper(*a, **kwargs):
+  >         f = o(*a, **kwargs)
   >         return singlebyteread(f)
   >     return wrapper
   >