I am quite new to Git, thus maybe I am missing something here.
dan@one:/var/www/$ git status -s
git diff --cached - it compares the HEAD and index, ie stuff added for commit.
git diff is between index and working directory, so if all the changes have been staged for commit, you won' see anything in
The third form is
git diff <commit> which compares working directory and commit. So doing
git diff HEAD will also give you the diff that you want as well.