Soak the mustard greens in water at least 1 hour or up to 4 hours, changing water occasionally. Drain and squeeze excess water out of the mustard greens.
Set a 14-inch wok or 5- to 6-quart pan over high heat. When pan is hot, add the oil and garlic. Stir-fry until garlic is lightly browned, about 15 seconds.
Add the pork and cook, stirring occasionally, until meat is lightly browned, 5 to 8 minutes.
Add the wine, mustard greens, kecap manis, sugar, and dark soy sauce.
Bring the pork mixture to a boil. Reduce the heat, cover and simmer, stirring occasionally, until the pork is tender when pierced, 1 ½ to 2 hours.
If liquid evaporates before the meat is tender, add a little water as needed, and continue simmering.
With a slotted spoon lift out pork and mustard greens and transfer to a serving dish.
Skim off the fat from pan juices and discard.
Taste juices and if too watery, boil, uncovered, over high heat until juices are reduced to about 1 cup.
Taste and add salt and more sugar, if needed. Pour over pork.
