Fix SetGarrisoned() call when transforming an entity

Before rP22753, the promotion code would carry the UnitAI IsGarrisoned flag, which the Transform helper incorrectly didn't do. However, this was missed when merging the two code paths.

This fixes an issue for example when units are promoted while atop walls.

Reported By: faction02

Patch By: Freagarach

Differential Revision: https://code.wildfiregames.com/D2303