The idea for this patch emerged from the discussion in D2846.
Nisean war horses were reserved for the heaviest shock cavalry (i.e. champion spearmen in 0 A.D.) This patch makes the `special_war_horses.json` technology similar to the `spartans_agoge.json` technology:
* increased health bonus to 25%, but added 10% training time;
* instead of all cavalry, limited to Champion Cavalry Spearman;
** increased health bonus to 25%, but added 10% food cost;
* cost raised to 6400 food, 3200 metal.
(For comparison, `upgrade_mace_silvershields.json` costs 10600 metal.)