If I run 'git status' on my repo it gives:
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
# modified: myfile
Your file is already staged to be committed. You can show it's diff using the
--cached option of git.
git diff --cached myfile
To unstage it, just do what git status suggests in it's output ;)
You can check The Git Index For more info.