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It would probably be quicker to replace the cable. Just remember to lube it and cover the tiller to prevent the rain from washing all oil or graphite out and rusting it tight again.
I have removed and soaked such cables in WD40 for a few days. After the soaking treatment - some rags and a pair of pliers will allow you to get it working (or break it). It usually works, but the amount of rust that is released shows how much damage has been done to the metal. A couple broke soon after re-installing. Now, I tend to replace first - and store such tools in the garage. Humidity can still rust them, but not nearly so much damage.