Source branch develop or target branch develop is not allowed

## Issue

### Context
- **Gitlab plugin version**: 1.5.13
- **Gitlab version**: 12.3.2
- **Jenkins version**: 2.198
- **Job type**: Pipeline
- **Gitlab webhook**: http://jenkins_user:jenkins_token@jenkins_ip/project/my_pipeline
- **Gitlab webhook properties**: {"push_events": true, "push_events_branch_filter": "develop"}

### Logs & Traces
Gitlab plugin log: Source branch develop or target branch develop is not allowed
### Problem description
- what were you doing: simple push
- what was expected: job was triggered
- what occurred finally: job was not triggered, got line Source branch develop or target branch develop is not allowed

What does it mean?