Full answer to the original question that was talking about a possible different path on local and remote is below
git fetch origin
git diff master -- [local-path] origin/master -- [remote-path]
Assuming the local path is docs/file1.txt and remote path is docs2/file1.txt, use git diff master -- docs/file1.txt origin/master -- docs2/file1.txt
This is adapted from GitHub help page here and Code-Apprentice answer above