Ensure downstream jobs use same sha for mergebranch.

Before, we wrongly did this:

  - build scala from the pr, merged into $mergebranch.
  - [time passes, $mergebranch advances]
  - to run the test suite downstream, merge the pr into $mergebranch

However, new tests may have appeared that fail with the original build of scala.
Thus, serialize the sha for $mergebranch in step 1, use that downstream.