Add "is not" comparison to the if template tag

[Alasdair Nicol]

#26118 added the is comparison to the if template tag. We should add the is not comparison as well.

From a quick test, it seems that {% x is not None %} is currently evaluated as x is (not None), instead of x is not None.

Discussion on django-developers mailing list: ​https://groups.google.com/d/msg/django-developers/jYdYCAtHHdw/WJYNXqAzEwAJ

[Alasdair Nicol]

Pull request ​https://github.com/django/django/pull/6436/files

[Tim Graham <timograham@…>]

In c16b9dd:

Fixed #26479 -- Added 'is not' operator to the if tag.

[Alasdair Nicol]

Re-opening because I think we should we document (and add tests) for when either or both of the variables are not in the template context.

As an aside, I can't see any tests for the != operator in the ​test_if.py. If I haven't overlooked them and they are missing, we could open a separate ticket to add them.

[Tim Graham]

#17664 "{% if %} template tag silences exceptions inconsistently" and a couple related tickets might be relevant to that.

[Alasdair Nicol]

New pull request: ​https://github.com/django/django/pull/6532

I had a quick look at #17664, but I wasn't sure what I should do for this ticket. If more changes/test cases related to unhandled exceptions are required, I think somebody else will have to take over the ticket.

[Tim Graham <timograham@…>]

In dac075e:

Refs #26479 -- Documented is/is not if tag operator behavior for nonexistent variables.