Set default args for BP_Groups_Group::__construct()

Must ensure that the 'populate_extras' param defaults to true when the class
is instantiated directly.

Fixes member counts in navigation.

Fixes #5411

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7934 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
POT, generated from r7934

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7935 cdf35c40-ae34-48e0-9cc9-0c9da1808c22