Allow force_insert for fixtures loading.

[Piotr Czachur]

There are some useful cases where silent overwriting is not desired: for example in unit testing, when you use multiple fixture files, and can duplicate PK by mistake. It has another advantage - django won't be checking if instance already exists in database, so test fixtures will load a bit faster.

This issue is not only connected with unit testing, I just give you practical example.

[Russell Keith-Magee]

#14989 was a closely related report, with a slightly different presentation.

I'm not convinced the "duplicate PK over multiple fixtures" is a significant problem -- in fact, you can use the existing behavior to your advantage (fixture load order can be used to determine which objects override others).

The other interpretation of this ticket -- the "only insert if it doesn't already exist" use case -- is essentially the same as "only load initial_data on first run", which is covered by #9549. The use of force_insert is an implementation issue.

Closing as a duplicate of #9549.