How do I mock the `message` variable with out refrencing the mock in the function via Java Spock?

Question

The bellow code works. But referencing the mock in the function smells wrong. (but my scent of code-smell could be wrong as well.).

Is there a way to call the mocked variable without referencing it in the function?

Function.

#!/usr/bin/env groovy

def call(String commitId) {
    String message = sh(
        script: "git show -s  --format=%s ${commitId}",
        returnStdout: true
    )
    if (message == null) message = messageMock
    echo "Based on the commit message: \n ${message.trim()}"
    switch ("${message.trim()}") {
        case ~/^(?i)(.*Feature.*)/:
            verType = 'minor'
            echo "The Next version will be a ${verType} type"
            break
        case ~/^(?i)(.*(Hot|Bug))fix.*$/:
            verType = 'patch'
            echo "The Next version will be a ${verType} type"
            break
        case ~/^.*Release.*$/:
            verType = 'major'
            echo "The Next version will be a ${verType} type"
            break

        default:
            verType = 'cosmetic'
            echo 'cosmetic changes only no new version'
            break
    }
    return verType
}

The unit test via Spock

package com.foo.jenkins

import com.foo.jenkins.testing.JenkinsPipelineSpecification

class SemVerSetTypeSpec extends JenkinsPipelineSpecification {


    def "expected minor value"() {
        setup:
        def SemVerSetType = loadPipelineScriptForTest("vars/semVerSetType.groovy")
        SemVerSetType.getBinding().setVariable("messageMock", "Feature Test")
        when:
        def verType = SemVerSetType("Feature")
        then:
        1 * getPipelineMock("sh")([script: "git show -s  --format=%s Feature", 'returnStdout': true])
        expect:
        "minor" == verType;
    }
}