How do you imitate the Civ 5 bonus fighting in forests promotion effect as a unit perk?

[Galgus]

Rather specific question, but I think using still-functional leftover promotions from Civ 5 in BE causes unit icon and upgrade screen bugs.

That's an acceptable cost for the effect to me, but I'd like to polish two mods more if I can.

Aliens need to have the effect somehow, and I'm not sure how to make them choose unit upgrades to get the perk if given the option.