Fixed #27539 -- Made TransactionTestCase._pre_setup() clear the queries_log so it's less likely to overflow.

TransactionTestCase.assertNumQueries() fails in an overflow situation.

Thanks Tim Graham for the review and edits.

Mistake in 97c1931c.

Migrations will now be created for changes to limit_choices_to.

Follow up to ab89414f.

Fixed #28293 -- Fixed union(), intersection(), and difference() when combining with an EmptyQuerySet.

Thanks Jon Dufresne for the report and Tim Graham for the review.

Thanks Robin Anupol for the initial report and workaround.

Returning invalid bytestrings in __str__() is unlikely in Python 3.

Thanks Mariusz Felisiak, Tim Graham, and Adam Johnson for review.

Unused since 5008a4db.