Setting key to an existing index can skip forder step

Consider

```
DT = data.table(
  ID1 = rep(10:1, 10:1),
  ID2 = rep(1:10, 1:10)
)
setindex(DT, ID1, verbose = TRUE)
# forder took 0 sec
setkey(DT, ID1, verbose = TRUE)
# forder took 0 sec
# reorder took 0 sec
```

I believe the second `forder` is superfluous -- only the physical reordering is necessary in the `setkey`.