Use GitPython library

[hadishakir]

• to select only tracked files
• works with .gitignore

[stuartleach]

This worked great for me. Thanks!

[shanecp]

I've checked the above commit. This creates a dependency on Git repos. It throws an exception as git.exc.InvalidGitRepositoryError, when the code is run on a project without an initiated repo.

I guess it would be good to have the ability to run code on a non-git repo as well (the current behaviour). It might be worthwhile to make the code detect if it's a git repo or not automatically, or have a flag to override it.

Another use case is the ability to run it on sub-folders with limited files (to get around the current token limit)