Cosmetics: Add TODO: prefix to todo.txt do message on already done task.

tests/t1500-do.sh

@@ -81,6 +81,6 @@ test_todo_session 'fail multiple do attempts' <<EOF
 TODO: 3 marked as done.
 
 >>> todo.sh -a do 3
-3 is already marked done
+TODO: 3 is already marked done.
 EOF
 test_done

todo.sh

@@ -966,7 +966,7 @@ case $action in
                 echo "TODO: $item marked as done."
 	    fi
         else
-            echo "$item is already marked done"
+            echo "TODO: $item is already marked done."
         fi
     done