[1.6.7] Garkul's Revenge has wrong learn/unlearn effects

[Erenion]

The Garkul's Revenge prodigy has a typo in its learn/unlearn functions.
Instead of increasing the damage you deal to humanoids and to giants, it increases the damage dealt to humanoids once, then sets it to the damage increase against giants (or usually 0) and increases it again.
This results in getting the expected 20% increase, but only against humanoids.

The bigger problem is in the unlearn function - the same happens here, leading to the following:
- the skill is unlearned
- your damage increase against humans is reduced by 20%
- your damage increase against humans is set to the "against giants" increase (or 0, which it usually should be)
- it is reduced by 20% again, leading to you now having -20% damage against humans

This causes problems when using the Writhing Ring of the Hunter (and also means you do not get the "against giants" increase when using the prodigy normally)