I was able to squash several commits after multiple merges from the master branch using the strategy found here: https://stackoverflow.com/a/17141512/1388104
git checkout my-branch # The branch you want to squash
git branch -m my-branch-old # Change the name to something old
git checkout master # Checkout the master branch
git checkout -b my-branch # Create a new branch
git merge --squash my-branch-old # Get all the changes from your old branch
git commit # Create one new commit
You will have to force an update if you need to push your squashed branch to a remote repository that you have previously pushed to, e.g. git push origin my-branch -f