Make captureOnCommitCallbacks capture callbacks added by callbacks

[Evgenii Morozov]

Sometimes there's a need to add additional on_commit callback from on_commit callback. For example, on_commit callback saves some model, but post_save handler for that model also uses on_commit callback to perform some additional action (e.g. spawn celery task). Current Django TestCase code ignores new callbacks added during on_commit callback execution.

I present a simple solution for this problem:
​https://github.com/django/django/pull/14796

[Mariusz Felisiak]

Sounds reasonable. Thanks!