commands: add simple spack commands --update-completion argument (#14607)

Instead of another script, this adds a simple argument to `spack
commands` that updates the completion script.  Developers can now just
run:

    spack commands --update-completion

This should make it simpler for developers to remember to run this
*before* the tests fail.  Also, this version tab-completes.

share/spack/qa/update-completion-scripts.sh

@@ -1,23 +0,0 @@
-#!/usr/bin/env bash
-#
-# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
-# Spack Project Developers. See the top-level COPYRIGHT file for details.
-#
-# SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-# Updates Spack's shell tab completion scripts
-
-# Switch to parent directory
-QA_DIR="$(dirname "${BASH_SOURCE[0]}")"
-cd "$QA_DIR/.."
-
-# Update each shell
-for shell in bash # zsh fish
-do
-    header=$shell/spack-completion.in
-    script=spack-completion.$shell
-
-    rm -f $script
-    spack commands --aliases --format=$shell --header=$header --update=$script
-    chmod +x $script
-done

share/spack/spack-completion.bash

@@ -507,7 +507,7 @@ _spack_clone() {
 _spack_commands() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -a --aliases --format --header --update"
+        SPACK_COMPREPLY="-h --help --update-completion -a --aliases --format --header --update"
     else
         SPACK_COMPREPLY=""
     fi