gitlab: Ensure to rebuild open merge requests on push events
requested to merge anlambert/swh-jenkins-jobs:gitlab-trigger-open-merge-request-push-both into master
Since recently, merge requests are not rebuilt by Jenkins when pushing new commits to it. This could be related to some default configuration changes in latest release of GitLab plugin for Jenkins.
As that behavior is not enabled by default in gitlab trigger settings explicitly enable it in jenkins jobs configuration.