Django Evolution 2.0 regressed adding non-NULL fields to a table

containing existing data. Previous versions would set the column to

allow NULL value in the temporary table, copy data over to it, create

the destination table with the current NULL flag, and copy data back.

2.0 changed this to follow the modern, recommended approach of creating

the proper schema for the temporary table, but that meant that the

column could no longer be left empty.
We now set the initial value as part of the INSERT statement, instead

of in a follow-up UPDATE statement. This is the same approach Django

chose, confirming that it's sound.
Some additional logic has been added to work around a data loss bug

that's been present (but never encountered in our use) for many years.

When going from a NULL to a non-NULL, we'd override any copied data with

initial data. Definitely not good. We now use the copied data if

available, and set an initial value if not.

All unit tests pass for all versions of Python and Django.