How to Diff between local uncommitted changes and origin

Given that the remote repository has been cached via git fetch it should be possible to compare against these commits. Try the following:

$ git fetch origin
$ git diff origin/master

I know it's not an answer to the exact question asked, but I found this question looking to diff a file in a branch and a local uncommitted file and I figured I would share

Syntax:

git diff <commit-ish>:./ -- <path>

Examples:

git diff origin/master:./ -- README.md
git diff HEAD^:./ -- README.md
git diff stash@{0}:./ -- README.md
git diff 1A2B3C4D:./ -- README.md

(Thanks Eric Boehs for a way to not have to type the filename twice)