Checkout step doesn't respect the parameters

### Version report

Jenkins and plugins versions report:

```
Jenkins 2.277.2
GitLab Plugin 1.5.20
Git Plugin 4.7.1
Git client plugin 3.7.1
(Up to date at the time of writing, only listing the probable culprits)
```

- What Operating System are you using (both controller, and any agents involved in the problem)?

```
Windows (10, and server 2019)
```

### Reproduction steps

Simply put I have this pipeline:
```
pipeline {
agent any
stages {
stage('Checkout') {
steps {
checkout([$class: 'GitSCM', branches: [[name: 'refs/remotes/origin/master']],
extensions: [[$class: 'CloneOption', depth: 10, noTags: true, reference: '', shallow: true],
[$class: 'RelativeTargetDirectory', relativeTargetDir: 'repo'],
[$class: 'LocalBranch', localBranch: 'merge_result'],
[$class: 'CleanBeforeCheckout'], [$class: 'PruneStaleBranch']],
userRemoteConfigs: [[credentialsId: '*****', name: 'origin', refspec: '+refs/heads/master:refs/remotes/origin/master', url: 'git@gitlab.************']]])
}
}
}
}
```

### Results

Expected result:

Checked out merge_result on origin/master both when launched via GitLab plugin and or manually.

Actual result:

When launched manually I have checked out local branch merge_result on origin/master.
But when launched via GitLab plugin (via a comment on MR) no matter what is in checkout() params the local merge_result is on origin/$gitlabSourceBranch.

I couldn't find the described behavior in either GitLab plugin or Git plugin documentation (thus reporting it as bug) neither is this behavior desired for us.