Production of troops when upgrade becomes available

For example, say you are in the process of building a warrior when the swordsman technology becomes available. If you open the production queue in that town, you lose all the production points you had put toward the warrior/swordsman.

Start by reproducing the issue through a town's production queue while a unit upgrade becomes available. Trace the production-queue entry point involved in opening the queue and verify that accumulated warrior production is preserved when it becomes a swordsman.

Written by the indexing model from the issue text.