Git commits in Moodle 3.6.4 by Paul Holden

commit e7bc8b35ee4537c7f3a82482c80b3ec2f57ee979
Author: Paul Holden <pholden@greenhead.ac.uk>
Date:   Wed, 27 Mar 2019 20:01:42 +0800
Tag:    v3.6.4

MDL-65197 editor: handle empty user preference in privacy export.

commit 13898c79db67d3470faf1d8c0d4fc99e1b37eee7
Author: Paul Holden <pholden@greenhead.ac.uk>
Date:   Fri, 08 Mar 2019 22:40:41 +0800
Tag:    v3.6.4

MDL-46931 roles: retrieve distinct users in sub-query.

commit ed88c64a5c407e0fdaf1c19e34cc2762bdf54b4a
Author: Paul Holden <pholden@greenhead.ac.uk>
Date:   Fri, 08 Mar 2019 20:32:24 +0800
Tag:    v3.6.4

MDL-43130 access: fix user counting when retrieving assignable roles.