this patch fixes and issue during initgroups in AD forests. Please see
the commit message for details.
To reproduce this you can create a new user outside of CN=Users on the
forest root. The new user can be created in an existing container or in
a new OU container. Most important is that it is not a child of
CN=Users. In a child domain (it must be a child, domains with a
different base won't trigger the issue) create a user with the same
name. With this setup 'id user(a)forest.root' will not return the complete
list of group the user is a member of and the patch should fix this.