This commits lifts the restriction that the outermost atomic block must
be declared with savepoint=False. This restriction was overly cautious.
The logic that makes it safe not to create savepoints for inner blocks
also applies to the outermost block when autocommit is disabled and a
transaction is already active.
This makes it possible to use the ORM after set_autocommit(False).
Previously it didn't work because ORM write operations are protected
with atomic(savepoint=False).
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