[18Ardennes] Only offer first market share for exchange

If a public company had multiple shares in the bank pool then each one
was being offered in exchange for minor companies, giving duplicated
buttons. All the shares are identical, so just offer the first one for
exchange.

lib/engine/game/g_18_ardennes/step/buy_sell_par_shares_companies.rb

@@ -83,7 +83,7 @@ def sellable_companies(entity)
           #         exchange for the minor company.
           def exchangeable_pool_shares(corporation, ability)
             minor = ability.owner
-            shares = @game.share_pool.shares_by_corporation[corporation]
+            shares = @game.share_pool.shares_by_corporation[corporation].take(1)
             return [] if shares.empty?
             return shares if corporation.owner == minor.owner
             return shares if corporation.receivership?