Jenkins 2.x pipeline: how to get git details after checkout?

[thomas....@teamaol.com]

How do I get the git information right which were - in the past - provided by Jenkins Git plugin?

I have found one information here: https://github.com/jenkinsci/pipeline-examples/blob/master/pipeline-examples/gitcommit/gitcommit.groovy
You see that in the code below (extract).

I have the treeish (job parameter which refers to a git tag name or git branch name).

What I'm still missing ... let's say since last build there might have been (n=5) commits.
So I would like to have the "n" commit messages. I know how to get the last "n" log messages in Git ...

...but how do I know how many commits have been since last build?

I see from the stage view (left side, row labels) that the pipeline definitely knows about
the number of commits and even it knows the git details showing them in a tooltip when
I move with the mouse cursor above the label.

How can I get this information after the checkout?

Kind Regards,

Thomas

APPENDIX:

    // ...

    // Retrieving first 6 characters of the SHA Git commit id.

    def getCommit() {

        def command="git rev-parse HEAD > " + this.workspace + "/GIT_COMMIT"

        script.sh(command)

        script.readFile(this.workspace + "/GIT_COMMIT").take(6)

    }

    // ...

    stage 'Get Code'

    checkout([$class: 'GitSCM', branches: [[name: treeish]],

    doGenerateSubmoduleConfigurations: false,

            submoduleCfg: [],

            userRemoteConfigs: [[credentialsId: 'XXXXXXXXXXX_CREDENTIALS',

             url: 'ssh://git@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxt']]])