Add progress bar on top of the pane that floats on the left in diff view...

During big reviews it is very helpful to know how many files are still lying ahead :-) 

This CL adds progress bar that allows to quickly estimate this

![screenshot-20130718 173356](https://f.cloud.github.com/assets/131846/820193/b03a050e-efbf-11e2-9c73-11fd17e12e87.png)