Staging untracked files and then making more changes doesn't show the diff correctly #370

If I stage an untracked file with some content, then modify the content so that we have some changes not staged for commit, the staged content and the unstaged diff is not shown correctly in GitX staging area. Instead, GitX shows the entire file content as it is on disk in both Unstaged and Staged Changes columns.

To reproduce:

1. Create and save a new untracked file with some content.
2. Stage the untracked file. Do not commit.
3. Make and save some changes to the same file.
4. Checked the version of the file in both Unstaged Changes and Staged Changes in the Stage view.