Mercenaries are duplicated

[Stan]

Steps to reproduce

• Garrison mercenaries
• Do the upgrade to lvl2
• Notice you now have duplicated mercenaries outside the building and inside it.

Or see attached replay.

[Stan]

replay

[Silier]

no release blockers on backlog pls

[Freagarach]

This is a nice one.

It depends on the structure you garrison the mercenaries in. Which leads to the barracks. Which happens to also have a XP trickle.

Situation:

• Merc in barracks, which is trickling XP.
• OnValueModification -> Merc is being promoted, old merc still exists.
• The old merc is ejected from the barracks, thus removing the trickle aura, triggering another value modification message.
• That new value modification message triggers another promotion.
• Only then does the first ChangeEntityTemplate finish, setting this.promotedUnitEntity, which would cause any extra increased XP to be forwarded.

[Freagarach]

In 25439:

Fix duplicating promoting entities in XP trickling structures.

The entity received a value modif. message while promoting, triggering a second promotion.

Differential revision: ​https://code.wildfiregames.com/D3918
Fixes: #6162
Comments by: @Angen, @Stan