New Predefined Variables GITLAB_USER_NAME and GITLAB_USER_LOGIN

Description

Since version 8.12 GitLab supports the predefined variables GITLAB_USER_ID and GITLAB_USER_EMAIL. Currently I want to tag commits with the user that triggered a manual build step.

Proposal

Add two new predefined variables GITLAB_USER_NAME and GITLAB_USER_LOGIN that contains real name and login username of the user that is running the job.