mac_githash shell script causes Win32 exception on AppBuild from Windows

[TheTalisman]

Not to be unexpected, of course, attempting to execute the bash script, "mac_githash.sh", described in the book will cause a Win32 exception when the AppBuilder's Build process is called when running Unity on Windows.

I suggest the following validation check (added to the GitHash() method in the Builder class):

private static string GitHash (  )
        {
            if (Application.platform == RuntimePlatform.OSXEditor) {
                string command = batchPath + "/mac_githash.sh";
                string output = ExecuteCommand (command);

                // We trim the output to remove new lines at the end.
                return output.Trim();
            }
            else {
                return "Not Implemented for Builds from Windows";
            }
        }