Tiny: Fix bug in metric names in index file (first character stripped off)

I noticed that the metric names in index file were missing the first character. Here's an example. 

In [8]: dirpath='/opt/graphite/storage/whisper/systems/u/ubuntu'

In [9]: base='/opt/graphite/storage/whisper/'

In [10]: print(dirpath[len(base) + 1:])
ystems/u/ubuntu

In [11]: print(dirpath[len(base):])
systems/u/ubuntu