If your master branch of your fork has diverged from upstream master you can check your master by typing
git checkout master
git fetch upstream master
git merge --ff-only upstream/master # if this fails your master is not the same as upstream i.e. it won't fast-forward
To clean up your master try (Option 2 is simpler, Option 1 downloads less)
# (Option 1) Fix master from point of divergence
#WARNING: you will lose your changes to master