What does 's stand for in the bold sentence, and what's the meaning of "that's you done"? Could you also provide me with a reference?
Harry entered Madam Malkin's shop alone, feeling nervous. Madam Malkin was a squat, smiling witch dressed all in mauve. "Hogwarts, dear?" she said, when Harry started to speak. "Got the lot here -- another young man being fitted up just now, in fact. " In the back of the shop, a boy with a pale, pointed face was standing on a footstool while a second witch pinned up his long black robes. Madam Malkin stood Harry on a stool next to him, slipped a long robe over his head, and began to pin it to the right length. "Hello," said the boy, "Hogwarts, too?" "Yes," said Harry. "My father's next door buying my books and mother's up the street looking at wands," said the boy. He had a bored, drawling voice. "Then I'm going to drag them off to took at racing brooms. I don't see why first years can't have their own. I think I'll bully father into getting me one and I'll smuggle it in somehow." Harry was strongly reminded of Dudley. "Have you got your own broom?" the boy went on. "No," said Harry. "Play Quidditch at all?" "No," Harry said again, wondering what on earth Quidditch could be. "I do -- Father says it's a crime if I'm not picked to play for my house, and I must say, I agree. Know what house you'll be in yet?" "No," said Harry, feeling more stupid by the minute. "Well, no one really knows until they get there, do they, but I know I'll be in Slytherin, all our family have been -- imagine being in Hufflepuff, I think I'd leave, wouldn't you?" "Mmm," said Harry, wishing he could say something a bit more interesting. "I say, look at that man!" said the boy suddenly, nodding toward the front window. Hagrid was standing there, grinning at Harry and pointing at two large ice creams to show he couldn't come in. "That's Hagrid," said Harry, pleased to know something the boy didn't. "He works at Hogwarts." "Oh," said the boy, "I've heard of him. He's a sort of servant, isn't he?" "He's the gamekeeper," said Harry. He was liking the boy less and less every second. "Yes, exactly. I heard he's a sort of savage -- lives in a hut on the school grounds and every now and then he gets drunk, tries to do magic, and ends up setting fire to his bed." "I think he's brilliant," said Harry coldly. "Do you?" said the boy, with a slight sneer. "Why is he with you? Where are your parents?" "They're dead," said Harry shortly. He didn't feel much like going into the matter with this boy. "Oh, sorry," said the other, not sounding sorry at all. "But they were our kind, weren't they?" "They were a witch and wizard, if that's what you mean." "I really don't think they should let the other sort in, do you? They're just not the same, they've never been brought up to know our ways. Some of them have never even heard of Hogwarts until they get the letter, imagine. I think they should keep it in the old wizarding families. What's your surname, anyway? But before Harry could answer, Madam Malkin said, "That's you done, my dear," and Harry, not sorry for an excuse to stop talking to the boy, hopped down from the footstool. "Well, I'll see you at Hogwarts, I suppose," said the drawling boy.