backport.py: Restore included_in branches check

• Restores check for included-in branches, which is used when confirming backports in the case a dependency was previously merged into a non-production branch, but the resulting commit eventually got included in a production branch. For example:
  1. Change A to operations/mediawiki-config master depends on Change B to mediawiki/core master
  2. Change B is merged into master and subsequently a train branch is cut and deployed.
• Adds integration test for included-in branches check
• Errors out when a change for backport is not deployable and removes any related integration tests
• In wait_for_changes_to_be_merged, checks revision number directly instead of revisions[revision]["_number"]

Related MRs: train-dev!71 (merged)

Bug: T360291 Bug: T362987