help: show help text for tapped external command

Previously, running e.g. `brew help bundle` when homebrew-bundle was not
tapped would tap homebrew-bundle and then run `brew bundle` showing
`Error: No Brewfile found` instead of the expected help text.

Library/Homebrew/brew.rb

@@ -143,6 +143,7 @@ begin
       safe_system(*tap_commands)
     end
 
+    ARGV << "--help" if help_flag
     exec HOMEBREW_BREW_FILE, cmd, *ARGV
   end
 rescue UsageError => e