Fix zsh nested subcommand option completion

In zsh completion context, the words array excludes the command name, so
subcommands are not matched and per-subcommand options are not
suggested.

To fix that, use `$words[1]` instead of `$words[2]` when dispatching
nested subcommands in generated zsh completions.

For reference, see zshcompsys's manpage [1]:

    $ man zshcompsys | col -bx | grep -F -A 13 '*::message:action'
                  *::message:action
                  *:::message:action
                         This describes how arguments (usually non-option
                         arguments, those not beginning with - or +) are to be
                         completed when neither of the first two forms was
                         provided.  Any number of arguments can be completed in
                         this fashion.

                         With two colons before the message, the words special
                         array and the CURRENT special parameter are modified to
                         refer only to the normal arguments when the action is
                         executed or evaluated.  With three colons before the
                         message they are modified to refer only to the normal
                         arguments covered by this description.

Also confirmed that this issue is zsh-specific. Bash and fish
completions work fine.

[1]: https://zsh.sourceforge.io/Doc/Release/Completion-System.html

Author: ZhongRuoyu

Library/Homebrew/completions.rb

@@ -435,7 +435,7 @@ def self.generate_zsh_nested_subcommand_completion(command, subcommands)
               _describe -t subcommands 'subcommand' subcommands
               ;;
             args)
-              case "$words[2]" in
+              case "$words[1]" in
         #{subcommand_cases.chomp}
                 *) ;;
               esac

Library/Homebrew/test/completions_spec.rb

@@ -464,6 +464,7 @@ def stub_nested_completion_command(command, subcommands)
         stub_nested_completion_command(nested_completion_command, nested_completion_subcommands)
         completion = klass.generate_zsh_subcommand_completion(nested_completion_command)
 
+        expect(completion).to include('case "$words[1]" in')
         expect(completion).to include("'1:subcommand:->subcommand'")
         expect(completion).to include("  _arguments -C \\\n    '--global[Use the global test file]' \\\n    " \
                                       "'1:subcommand:->subcommand'")

completions/zsh/_brew

@@ -416,7 +416,7 @@ _brew_analytics() {
       _describe -t subcommands 'subcommand' subcommands
       ;;
     args)
-      case "$words[2]" in
+      case "$words[1]" in
 state)
         _arguments \
           '--debug[Display any debugging information]' \
@@ -679,7 +679,7 @@ _brew_bundle() {
       _describe -t subcommands 'subcommand' subcommands
       ;;
     args)
-      case "$words[2]" in
+      case "$words[1]" in
 sh)
         _arguments \
           '--check[Check that all dependencies in the Brewfile are installed before starting the shell. Enabled by default if `$HOMEBREW_BUNDLE_CHECK` is set]' \
@@ -981,7 +981,7 @@ _brew_completions() {
       _describe -t subcommands 'subcommand' subcommands
       ;;
     args)
-      case "$words[2]" in
+      case "$words[1]" in
 unlink)
         _arguments \
           '--debug[Display any debugging information]' \
@@ -1155,7 +1155,7 @@ _brew_developer() {
       _describe -t subcommands 'subcommand' subcommands
       ;;
     args)
-      case "$words[2]" in
+      case "$words[1]" in
 state)
         _arguments \
           '--debug[Display any debugging information]' \
@@ -2056,7 +2056,7 @@ _brew_services() {
       _describe -t subcommands 'subcommand' subcommands
       ;;
     args)
-      case "$words[2]" in
+      case "$words[1]" in
 stop|unload|terminate|term|t|u)
         _arguments \
           '(--file)--all[Stop all services and unregister them from launching at login (or boot), unless `--keep` is specified]' \