Currently the only way to do a heavy weight checkout is to set a system property for Jenkins java arguments. (viewable only from source code)
'-Dorg.jenkinsci.plugins.workflow.multibranch.SCMBinder.USE_HEAVYWEIGHT_CHECKOUT=true'

You would think this would be configurable per job, like it is in single pipeline job; not on a global scale via a Jenkins java argument.

This change was introduced with -JENKINS-33273-.

Reason for this – I want to be able to do a PreBuildMerge action on the Jenkinsfile so the pipeline is up to date and tested with latest changes. Otherwise you are building/testing with an old pipeline which may fail after it's been accepted/merged.

Note: [If needed for testing] With latest plugins, Git PreBuildMerge extension is not valid with Multibranch Pipeline anymore. See Git-Plugin PR 513 https://github.com/jenkinsci/git-plugin/pull/513 to test with.