Fix group modification idempotency

Currently, if you modify the groups for a user and include the user's namesake group, you will always get a 'changed' notification, whether or not that is true.

A concise demo:
````
---
- user: name=bob
- user: name=bob groups=bob # bob is not explicitly a member of bob initially
- user: name=bob groups=bob
  register: group_modify

- assert:
  that:
  - not group_modify|changed
````

This is just #7028 but with the commit message cleaned up.