Git commits in Moodle 3.6.0 by Adrian Perez

commit ac33483290931f8d6cef8f81cee46392310e55f2
Author: Adrian Perez <adrian.perez@ffhs.ch>
Date:   Fri, 20 Jul 2018 20:25:25 +0800
Tag:    v3.6.0-beta

MDL-63002 core_grade: fixed deletion of overriden letters