A pipeline job with SCM branch wildcards does not show branch info in the build description

## Issue

### Context

- **Gitlab plugin version**: 1.4.5
- **Gitlab version**: 9.0
- **Jenkins version**: 2.63
- **Job type**: Pipeline

### Problem description

A freestyle job with SCM branch wildcards (like `*/**`) shows the action type, branch, and source/target branch for merges in the build description automatically.


The same does not seem to happen with pipeline jobs.

I could probably construct something similar, but it's less than perfect (shown here, using the declarative pipeline syntax).

```
script {
currentBuild.description = "${build description string from env vars here}"
}
```