Fix group modification idempotency

##### Issue Type: Bugfix Pull Request
##### Ansible Version: 1.6.3, devel
##### Environment: Ubuntu 12.04, probably more generally applicable
##### Summary:
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.

This is just #7028 but with the commit message cleaned up.
##### Steps To Reproduce:
````
---
- 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
````
##### Expected Results:
The assert succeeds, and the task file finishes successfully.
##### Actual Results:
The assert fails