Tests by Tim Graham, report from jeroen.pulles@redslider.net.

If LEFT JOINs are required for correct results, then trimming the join
can lead to incorrect results. Consider case:

TBL A: ID | TBL B: ID  A_ID
       1           1   1
       2
Now A.order_by('b__a') did use a join to B, and B's a_id column. This
was seen to contain the same value as A's id, and so the join was
trimmed. But this wasn't correct as the join is LEFT JOIN, and for row
A.id = 2 the B.a_id column is NULL.

There was an old complaint about nullable one-to-one field cascading
even when the o2o field was saved to None value before the deletion.
Added an test to verify this doesn't happen.

Also some PEP 8 cleanup.

Conflicts:
	docs/ref/django-admin.txt

The default delete action resulted in a NoReverseMatch if it were to
list any Model with a ModelAdmin with `get_urls` overridden to remove
the change url.  Catching the error and not displaying the link in that
case, as was already done for models with no registered admins.

Thanks Keryn Knight for the report.

Caused by commit 7737305a

The EmptyResultSet wasn't treated correctly so the end results was
incorrect, too. The bug had been already fixed in master so only tests
added.

There were clauses that weren't needed in the subqueries. These were
ORDER BY, SELECT FOR UPDATE and related selections.

An isnull lookup produced incorrect SQL. This was already fixed
earlier, so only tests added.

Added proxy_models tests by Harm Geerts <github@geertswei.nl>.

Fixed #11892, fixed #16458, fixed #14492.

Proxy __len__ and __contains__ for LazyObject

Python 2.7 allows to combine several 'with' instructions.

Thanks to Collin Anderson for the suggestion and Tim Graham for
reviewing the patch.

Thanks AtomicSpark for the report.

Forwardport of 61283a82 from 1.5.x

Follows 3f2befc9

Thanks clay.evil@ for the suggestion.

Thanks Jug_ for suggestion, john_scott for the initial patch,
and Tim Graham for code review.