No files merged may result in bogus merge conflict error

So I found an interesting edge-case here: When cherry-picking a commit which has previously been applied and having untracked files in the working directory always resulted in a merge conflict error.

The reason for this is that even though we didn't merge any changes (they already exist in the index), we were passing 0 as the count parameter to ```git_diff_index_to_workdir```, resulting in all of the untracked files being reported as conflicted.

I've added a test case for this, the most natural place I found was using ```git_cherrypick``` as this is the most likely place you'll hit the problem.
Ok, I'll drop this change and maybe then look at another approach. The reasoning I had here is that I felt the message didn't really didn't tell me where the conflict was coming from.

Had a look at the index, everything looked cool, looked at the working directory, nothing should have conflicted. Finally modified the error message which pointed out it was an untracked file that should not have caused the problem. (Just my reasoning, I'll drop it regardless).