cmd/fetch: restore --HEAD functionality

[EricFromCanada]

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---

brew fetch was always downloading the stable spec. This fixes that.

[reitermarkus]

Are there any formulae that have a different head depending on OS/arch? If not, maybe --HEAD should conflict with --os and --arch.

[apainintheneck]

Are there any formulae that have a different head depending on OS/arch? If not, maybe --HEAD should conflict with --os and --arch.

It doesn't look like there are any formulas in core that have multiple head definitions.

$ git grep -El ' head "| head do' | awk '
{ files[$0]++ }
END {
  total = 0
  for(file in files) {
    if(files[file] > 1) total++ 
  }
  print total
}'
0