Jenkins: how to build Gerrit changeset without Gerrit plugin

1. To to job config
2. Update the Git plugin settings:
  - in Advanced --> Refspec set `refs/changes/*:refs/changes/*`
  - in 'branch' set the Gerrit changeset ref, such as `refs/changes/17/417/2` (you can pick it from 'Download' dropdown menu in Gerrit changeset window)