Fix regression on after_commit in nested transactions.

after_commit should not run in nested transactions, however they should
run once the outermost transaction gets committed. This patch fixes the
problem copying the records from the `Savepoint` to its parent. So the
`RealTransaction` will have all records that needs to run callbacks on it.

[fixes #16425]

review @rafaelfranca @chancancode
this should be applied on 4-0-stable too.
I will work on a different fix for master, as the code is different.