Git commits in Moodle 3.6.3 by Justus Dieckmann

commit d783d4759c22c4ed1e8f72f832d668069424f71f
Author: Justus Dieckmann <justusdieckmann@wwu.de>
Date:   Fri, 21 Dec 2018 03:19:38 +0800
Tag:    v3.6.3

MDL-58256 group: prevents users to be added to a group twice