Investigate possible race conditions when gitsync-ing DAG submodules

[adwk67]

Description

As a user of Airflow I want to be sure that DAGs that use submodules are seamlessly updated and registered with Airflow via gitsync. This covers the following:

Tasks

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1. Check that DAGs are consistent i.e. that the state when the DAG is started is the same despite subsequent syncs while the DAG is still running
2. Check that "new" submodules are correctly loaded/registered