Deal with include_prior_dates

[dstandish]

Body

We must think through what the behavior should be in xcom_pull for include_prior_dates.

I think xcom pull is the only place where we need to deal with something like this, but there could be others.

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.

[bil-ly]

Can i look into this?