BackfillJob overrides start date but that doesn't propagate to executor commands

[jlowin]

I'm frankly not sure if this is a bug or not but it's weird so I'm creating a tracking issue for it. This will probably be addressed as part of either the #1225 resolution or the start_date roadmap item.

BackfillJob overrides the start date of a dag and its tasks. However, the override doesn't carry through to the executor commands. For example, SequentialExecutor issues an airflow run command and loads the same dag with its original start date.

This creates a problem because BackfillJob will always think the first iteration of a task with depends_on_past=True is runnable, since the execution date and start date match. But the executor it assigns to run that task will conclude that it it NOT runnable, because execution date and start date don't match.

[jlowin]

Closed in #1271