check distinct(user) based on just user_id

check distinct(user) based on just user_id

The logic to display the Check(s) on the patch list wants to really do a
DISTINCT(user_id,context) ORDER BY DATE query, but with Django that is
currently a bit Too Hard (at least for me).

But what we can do is from python just use the user_id rather than the
user object itself. Same functionality, no join or prefetching users.

This saves a couple of hundred queries on the linuxppc/list/ view and
makes loading it about 4x faster in terms of time spent in the db.

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>