The difference is subtle. "Till you finish it" puts the emphasis on the action, or the event, of finishing the work. In other words "till you perform the action of finishing the work." "Tіll you have fіnіshed іt" is in the present perfect tense, so it puts the emphasis on the state you are in, or the quality you possess, upon having finished the work. This state, or property, is that of "a person who has finished their work." An enviable state/property. :) That's the difference, but most people would say that since the difference doesn't alter *when* you can stop working, it's not a very important difference. :)

There is no real difference, the tenses are different but the meaning is the same. I would write it like 'You mustn't stop working until you've finished' or '...'til you've finished.'

In this context both mean exactly the same.

You can't stop working till you finish it.