Fixes for GitFileVersionsController

I had some issues using git with fitnesse, so I decided to look at the source code to see if I could fix them and contribute to the project. 
The proposed fixes solved the following problems:
1) IndexOutOfBouldsException: when pagePath=workTreePath, adding 1 to the length of workTreePath was causing substring() to throw that exception.
2) The author of the commits to the git repository was the user who executed fitnesse and not the user logged on (we have secure-write enabled).