By default, for PR builds, this should be done BEFORE parsing .circleci/config.ymlgit checkout $CIRCLE_PR_TARGET_BRANCHgit merge $CIRCLE_BRANCHThere should be an option to opt out of this on a per repo basis. Travis CI does this by default:https://docs.travis-ci.com/user/pull-requests/#how-pull-requests-are-builtfrom their docs:Rather than build the commits that have been pushed to the branch the pull request is from, we build the merge between the source branch and the upstream branch.related: https://ideas.circleci.com/ideas/CCI-I-894

CCI-I-1568