This completes to `spack concretize`:
```
spack conc<tab>
```
but this still gets hung up on the difference between `concretize` and `concretise`:
```
spack -e . conc<tab>
```
We were checking `"$COMP_CWORD" = 1`, which tracks the word on the command line
including any flags and their args, but we should track `"$COMP_CWORD_NO_FLAGS" = 1` to
figure out if the arg we're completing is the first real command.
f6b23b4653