env.BRANCH_NAME = "mycoolbranch"// BRANCH_NAME is predefined in multibranch pipeline job
env.J_GIT_CONFIG = "true"
env.J_USERNAME = "Jenkins CI"
env.J_EMAIL = "jenki...@example.com"
env.J_CREDS_IDS = '02aa92ec-593e-4a90-ac85-3f43a06cfae3' // Use credentials id from Jenkins
def gitLib = load "git_push_ssh.groovy"
...
gitLib.pushSSH(commitMsg: "Jenkins build #${env.BUILD_NUMBER}", tagName: "build-${env.BUILD_NUMBER}", files: "changelog.txt someotherfile.txt");
checkout scm