-stringWithFileSystemRepresentation instead of -stringWithUTF8String in GTDiffFile

According to Apple's documentation, we should be using `-[NSFileManager stringWithFileSystemRepresentation:length:]` since libgit2 gets the path using standard c library functions.


I've also added some assertions so that we fail here if we have a bad `git_diff_file`, but maybe we should just return `nil` instead?