programs/bash: install nix-bash-completions if completion is enabled

[hedning]

Motivation for this change

Enable completion for the Nix command line tools when bash completion is enabled.

The completion scripts should be pretty solid now, and I think it would be good for the user experience to install it automatically when the user have toggled programs.bash.enableCompletion.

Things done

Removed spotty nixops completion, which wasn't ready for prime time, from nix-bash-completions.

Added a note to the 18.03 changelog.

Tested VMs with nixos-rebuild build-vm configured with programs.bash.enableCompletion turned on and off.

cc @bjornfor

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[bjornfor]

Thank you! I love being able to complete Nix commands!

[Mic92]

Do we have those enabled on the live cd? I think we should.

[oxij]

Would be nice to do the same for zsh, I think.

[oxij]

Ah, ignore the previous comment. `nixos/modules/programs/zsh/zsh.nix` already does it.

[hedning]

@Mic92 programs.bash.enableCompletion is false by default, and it doesn't look like it's turned on explicitly in any of the cd configs.

I agree that it would be good to turn it on for the cd.