Explicitly notify user when sub commit diff has no changes.

There are some cases when git shows no changes between files, which are little strange and may need further investigation but below code explicitly handles that case

![screen shot 2014-03-29 at 22 27 50](https://cloud.githubusercontent.com/assets/5281068/2560302/a8a35b82-b7bb-11e3-8d53-1bf767c22d0d.png)