Add subdir parameter to dags reserialize command

[mik-laj]

Hi.
Today I looked at this command and noticed a few problems:

• The DagBag.collect_dags method is called two times. The first time through DagBag.__init__, and the second time explicitly in a command code. This is not needed and causes performance degradation.
• safe_mode has been overridden to false in the command code for no reason, which means more files are processed than needed.
• Parameter subdir is not supported, which is inconsistent with the rest of the commands that create the DagBag instance. Whenever a DagBag is created by any other commands, it is possible to set the dag_folder using the subdir parameter including airflow dag list, airflow scheduler, and others.

I had a problem with choosing a PR title that would look good in a changelog, because we have 3 very related problems here, but I think adding a new CLI parameter is the most user-facing, The rest of the problems would not be (probably) noticed by anyone without looking at the code.

CC: @collinmcnulty, @potiuk, @uranusjr, @sfc-gh-mkmak

Best regards,
Kamil Breguła

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[uranusjr]

Does this line need to be kept?

[mik-laj]

No, and I described in the PR description why it is not needed.

[uranusjr]

But the arguments here are different from the call in __init__.

[kaxil]

Since DagBag().file_last_changed will be empty, I think it won't make a difference if only_if_updated is set to True or False

[mik-laj]

only_if_update = False means the dags will be read again if the file has changed. In our case, we don't need to load the same files twice, so this parameter doesn't apply here.

[kaxil]

Test failure @mik-laj :

___________________ TestCliTasks.test_run_get_serialized_dag ___________________
  
  self = <tests.cli.commands.test_task_command.TestCliTasks object at 0x7f2bcf86d890>
  mock_local_job = <MagicMock name='LocalTaskJob' id='139826142040912'>
  mock_get_dag_by_deserialization = <MagicMock name='get_dag_by_deserialization' id='139826142042192'>
  
      @mock.patch("airflow.cli.commands.task_command.get_dag_by_deserialization")
      @mock.patch("airflow.cli.commands.task_command.LocalTaskJob")
      def test_run_get_serialized_dag(self, mock_local_job, mock_get_dag_by_deserialization):
          """
          Test using serialized dag for local task_run
          """
          task_id = self.dag.task_ids[0]
          args = [
              'tasks',
              'run',
              '--ignore-all-dependencies',
              '--local',
              self.dag_id,
              task_id,
              self.run_id,
          ]
  >       mock_get_dag_by_deserialization.return_value = SerializedDagModel.get(self.dag_id).dag
  E       AttributeError: 'NoneType' object has no attribute 'dag'
  
  tests/cli/commands/test_task_command.py:152: AttributeError