This quote is very telling of what was going. Jesus seemed to always be about forgiveness and even about loving thy enemy. He is asking god to forgive someone who is harming his son. Would god have forgiven if Jesus had not asked? Maybe maybe not but it is a clear attempt to bring about a forgiving god. If god can forgive for harming his own son god can forgive for anything but it doesn't mean that god couldn't have done it prior to sacrificing his son. I doubt it had to be that drastic as Jesus was already preaching of love and forgiveness and that quote only shows the same qualities he had been displaying throughout his ministry. It doesn't mean Jesus needed to be god to do this especially if he is asking God to do the forgiving.
Let's look at the two thief's on the cross with Jesus. Did they have any forgiveness in their heart? About the closest one came to was in stating that this man had done nothing wrong.
The point is that mankind can not forgive to the degree that God can. Jesus understood at that point the object of His mission, thus fulfilling all prophecy.
Isa 53:12 Therefore will I divide him a portion with the great, and he shall divide the spoil with the strong; because he hath poured out his soul unto death: and he was numbered with the transgressors; and he bare the sin of many, and made intercession for the transgressors.
The part in red, was the fulfillment of that prophecy when Jesus made the statement.
Blessings, AJ