I ran into this exploring JENKINS-20941

Steps to reproduce:
1) Install latest git plugin and git client plugin from experimental repo
2) In a workflow job from the snippet generator, choose 'General SCM'. Choose 'Git' as the type, then in the dropdown for additional behaviors, add 'Advanced Submodule Behaviors', then click 'Generate Groovy'. The generator outputs this which is syntatically incorrect:
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checkout([$class: 'GitSCM', branches: [[name: '*/master']], doGenerateSubmoduleConfigurations: false, extensions: [<object of type hudson.plugins.git.extensions.impl.SubmoduleOption>], submoduleCfg: [], userRemoteConfigs: [[credentialsId: '23a7815f-3fbd-462c-ba41-23668de4868d', url: 'git@git.example.com:foo/bar.git']]])
–
Expected is something more like:
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checkout([$class: 'GitSCM', branches: [[name: '*/master']], doGenerateSubmoduleConfigurations: false, extensions: [[$class: 'RelativeTargetDirectory', relativeTargetDir: 'src'], [$class: 'SubmoduleOption', disableSubmodules: false, recursiveSubmodules: true, trackingSubmodules: false, parentCredentials: true]], submoduleCfg: [], userRemoteConfigs: [[ url: 'git@git.example.com:foo/bar.git']]])
–
(The parentCredentials doesn't seem to work anyway yet, but it at least pareses as valid that way.)