help: print cli/parser help message if used

Library/Homebrew/brew.rb

@@ -94,7 +94,7 @@ begin
   # - if cmd is Cask, let Cask handle the help command instead
   if (empty_argv || help_flag) && cmd != "cask"
     require "help"
-    Homebrew::Help.help cmd, empty_argv: empty_argv
+    Homebrew::Help.help cmd, empty_argv: empty_argv, internal_cmd: internal_cmd
     # `Homebrew.help` never returns, except for external/unknown commands.
   end
 

Library/Homebrew/help.rb

@@ -72,6 +72,9 @@ module Homebrew
       # Resume execution in `brew.rb` for unknown commands.
       return if path.nil?
 
+      # Resume execution in `brew.rb` for external commands using "cli/parser".
+      return if !flags[:internal_cmd] && path.read.match?("require .cli/parser.")
+
       # Display help for internal command (or generic help if undocumented).
       puts command_help(path)
       exit 0