I have cloned a github repository and made no changes locally. Github repository moved forward with commits on the same branch.
1) Add any remote repositories you want to compare:
git remote add foobar git://github.com/user/foobar.git
2) Update your local copy of a remote:
git fetch foobar
Fetch won't change your working copy.
3) Compare any branch from your local repository to any remote you've added:
git diff master foobar/master