workaround for JENKINS-28447 in case of github trigger plugin.

JENKINS-28447.workaround.groovy

// Reconfigure job to have Pipeline DSL configured inside job, instead of from scm.
// Put snippet below to pipeline text box
// Define additional job parameters: 
// GITHUB_PROJECT: string, URL to your GitHub repository
// GITHUB_AUTH: string, credentials ID to use in case of private GitHub repository
node {
    dir('pipeline_handover') {
        checkout([$class: 'GitSCM', 
            branches: [[name: "origin-pull/$GITHUB_PR_NUMBER/$GITHUB_PR_COND_REF"]], 
            doGenerateSubmoduleConfigurations: false,
            extensions: [], 
            submoduleCfg: [],
            userRemoteConfigs: [
                [credentialsId: "${GITHUB_AUTH}",
                name: 'origin-pull',
                refspec: "+refs/pull/$GITHUB_PR_NUMBER/*:refs/remotes/origin-pull/$GITHUB_PR_NUMBER/*",
                url: "${GITHUB_PROJECT}"]]])
    }
    load 'pipeline_handover/Jenkinsfile'
}


// This part of example goes to Jenkins file inside your Git tree:
// Code below would checkout Git content in both scenarios for triggering builds 
// (pull requests, push to branch) and share same Jenkinsfile
node('docker') {
    stage 'Checkout own content'
    dir(current_project) {
        if (binding.variables.get("GITHUB_PR_NUMBER")) {
            // we are building pull request
            checkout([$class: 'GitSCM',
                branches: [[name: "origin-pull/$GITHUB_PR_NUMBER/$GITHUB_PR_COND_REF"]],
                userRemoteConfigs: [
                    [credentialsId: "${GITHUB_AUTH}",
                    name: 'origin-pull',
                    refspec: "+refs/pull/$GITHUB_PR_NUMBER/*:refs/remotes/origin-pull/$GITHUB_PR_NUMBER/*",
                    url: "${GITHUB_PROJECT}"]]])
        } else {
            checkout scm
        }
    }
    // ....
}