git-revert-file-from-upstream.txt

# Assuming you did not commit the file, or add it to the index, then:
git checkout -- filename

# Assuming you added it to the index, but did not commit it, then:
git reset HEAD filename
git checkout -- filename

# Assuming you did commit it, then:
git checkout origin/master filename

# Assuming you want to blow away all commits from your branch (VERY DESTRUCTIVE):
git reset --hard origin/master