ORM cleanup

[Anssi Kääriäinen]

Three items:

• Cleanup IS NOT NULL condition generation for negated lookups in add_filter() (also fixes one bug, and simplifies split_exclude()). Handled in commit: ​https://github.com/akaariai/django/commit/4d8ed818beb59b1e6d04eead6c9dad0bf2bdafb7
• Removal of Query.join()'s promote kwarg. This also improves handling of promotion in select_related(). Before select_related() always promoted any used join if nullable, now only if the join is newly created. In case there is already existing inner join we know any null value will remove results in any case, so the louter join is wrong here. Commit: ​https://github.com/akaariai/django/commit/f7f7044dedabdd04b6806409239e040c25d2bb83
• Cleaned up join promotion in combine(), commit: ​https://github.com/akaariai/django/commit/bfc88e7e3dded3d4259d70f31d65d36b0e197144

I plan to commit these shortly.

[Anssi Kääriäinen <akaariai@…>]

In 3fef304ff237fe692459c1f5b840acf7886c50bb:

Refactored negated IS NULL handling

This one cleaned up add_filter() negated filter generation. As a side
effect split_exclude() was cleaned up, too.

Refs #19849

[Anssi Kääriäinen <akaariai@…>]

In edf93127bf2f9dc35b45cdea5d39a1b417ab1638:

Removed join() promote kwarg

The join promote=True was over-aggressive in select_related handling.
After that was removed, the only other user was query.combine(). That
use case is very easy to handle locally, so there is no more need for
the join(promote=True) flag.

Refs #19849.

[Anssi Kääriäinen <akaariai@…>]

In 10f9ba046f6122414fe5501934b6e080c3554a85:

Cleaned up join promotion in query.combine() with OR

Refs #19849

[Anssi Kääriäinen <akaariai@…>]

In e8fc3f3783744e4fcd3ff0ac43e3ce2572292f3a:

Added a test for negated Q object querying

The added test is from the patch in ticket #19672 (written by Ian
Kelly). Fixed #19672, refs #19849.

[Anssi Kääriäinen]

I forgot to close this after the commits...