dev-cmd/contributions: Use a named arg for required `repositories`
dev-cmd/contributions: Use a named arg for required `repositories`

- This is apparently "more in-keeping with how we do required arguments
  (never requiring flags)" across Homebrew.
- New usage: `brew contributions me@issyl0.co.uk all`, `brew
  contributions me@issyl0.co.uk brew,core`

Homebrew/brew,nandub/brew,JCount/brew,MikeMcQuaid/brew,Homebrew/brew,nandub/brew,Homebrew/brew,Homebrew/brew,JCount/brew,nandub/brew,MikeMcQuaid/brew,nandub/brew,JCount/brew,JCount/brew,MikeMcQuaid/brew,MikeMcQuaid/brew