I know of the scene you mentioned, but this was probably an alternate take/rehearsal and not one that was going to be used in the movie and i don't think it was filmed because of what is seen in the movie it wouldn't fit in. Actually there is no continuity error in this scene. Luke has the detonator in his left in the last shot before he dives out of the way and again as he jumps out of the way of the AT-AT foot. Then it cuts to the command centre scene so Luke could easily have attached it during this time as he already has it with him.