Ensure --help and mix help go through the same path, closes #2142

Author: José Valim

lib/mix/lib/mix/cli.ex

@@ -10,7 +10,7 @@ defmodule Mix.CLI do
 
     case check_for_shortcuts(args) do
       :help ->
-        display_banner()
+        proceed(["help"])
       :version ->
         display_version()
       nil ->
@@ -89,10 +89,6 @@ defmodule Mix.CLI do
     end
   end
 
-  defp display_banner() do
-    run_task "help", []
-  end
-
   defp display_version() do
     IO.puts "Elixir #{System.version}"
   end