test_backport.py: Revised setup_depends_on_included_in()

setup_depends_on_included_in:

• Specify intentions in the docstring.
• Use the master branch of mediawiki/config instead of test-branch.
  This represents a realistic workflow.
• Wait for the master branch change to merge after approving it.
• Use a fake cherry-pick of the master branch change into the
  production branch. This avoids dealing with potential conflicts.

git_clone:
Replace git reset -q --hard FETCH_HEAD with
git checkout -q --force -B <branch> origin/<branch> to ensure that
we end up in a named branch.