Added sparse checkout support

Hi,

Here is my implementation of "Git Sparse Checkout" for CliGitApi.
http://git-scm.com/docs/git-read-tree

The implementation does not seem available yet in JGIT.

"Git Sparse Checkout" feature is really useful when dealing with multi-project Git repository. 
It can indeed save a lot of space. 
http://stackoverflow.com/questions/10791472/jenkins-and-git-sparse-checkouts

I also have a pull request for Jenkins Git Plugin that enables "Git Sparse Checkout". I will propose it if this one is merged.

Regards,

Thomas