Members of XWikiAllGroup's subgroups aren't automatically added as subwiki members

Steps to reproduce:

1. Login with an Admin
2. Create 2 global users: U1 & U2
3. Create a new subwiki: S1
4. Go to S1's Administration: Users and add U1 as a member
5. Go to S1's Administration: Groups and create a new group: G1
6. Edit G1 and add U2 as a member
7. Edit XWikiAllGroup and add G1 as a subgroup
8. Go to S1's Administration: Users and check the "Current Members" section -> only U1 is listed as a member
9. Go to S1's Main.UserDirectory -> again only U1 is listed as a member