Squash PR commits into a single commit

squash-pr-commits-to-one.txt

git fetch upstream
git checkout mybranch
git merge upstream/master

# if necessary, resolve conflicts and git commit
# ...

git reset --soft upstream/master
git commit -am 'Some cool description for a single commit'
git push -f

Note that it's super important that you merge before resetting, and that the argument is the same master branch. 
Otherwise you risk messing up your local history.