Contrary to what was said in the comment in the code, units were not ejected when upgrading towards a non garrisonHolder
- rP20939: Garrisoned units are lost after an upgrade towards a non garrison holder
Put some units on a wall and upgrade it to a gate
Added the backref, to the concern raised there, since this is the bug I mentioned in that revision (so when we commit this the concern should be closed). IIRC back then I checked that the revision was causing the bug, but I might be mistaken in that.