It would be useful if the merge commit sha1 - I.e. the b4af1bf184f8f2fa6e50a784dda17916d97dc629 from:
could be made available to downstream jobs through a build-trigger parameter (i.e. as an environment variable similar to ghprbActualCommit, etc.
The use-case for this is that your downstream job, say a test job, wants to be able to check-out the exact same code that was built into what is being tested.