Problem statement: you want to make sure that the PR branch is up-to-date with the target branch before running your CI job against it.
As a secondary concern, you don't want to run the job if there are conflicts with the target branch.
Solution:
# setup the github user
git config --global user.email $( git log --format='%ae' $CIRCLE_SHA1^! )
git config --global user.name $( git log --format='%an' $CIRCLE_SHA1^! )