The word repented in this text doesn't mean Job sinned. There are several scriptures in the bible that says God repented. However we know that God can not sin.
Example:
9 And the Lord said unto Moses, I have seen this people, and, behold, it is a stiffnecked people:
10 Now therefore let me alone, that my wrath may wax hot against them, and that I may consume them: and I will make of thee a great nation.
11 And Moses besought the Lord his God, and said, Lord, why doth thy wrath wax hot against thy people, which thou hast brought forth out of the land of Egypt with great power, and with a mighty hand?
12 Wherefore should the Egyptians speak, and say, For mischief did he bring them out, to slay them in the mountains, and to consume them from the face of the earth? Turn from thy fierce wrath, and repent of this evil against thy people.
13 Remember Abraham, Isaac, and Israel, thy servants, to whom thou swarest by thine own self, and saidst unto them, I will multiply your seed as the stars of heaven, and all this land that I have spoken of will I give unto your seed, and they shall inherit it for ever.
14 And the Lord repented of the evil which he thought to do unto his people. Exodus 32:KJV