commands: helper for official external commands.

Library/Homebrew/commands.rb

@@ -100,6 +100,14 @@ module Commands
     find_commands HOMEBREW_DEV_CMD_PATH
   end
 
+  def official_external_commands_paths
+    %w[bundle services].map do |cmd|
+      tap = Tap.fetch("Homebrew/#{cmd}")
+      tap.install unless tap.installed?
+      external_ruby_v2_cmd_path(cmd)
+    end
+  end
+
   def internal_commands
     find_internal_commands(HOMEBREW_CMD_PATH).map(&:to_s)
   end