patch applied (ghc): Help the user when she tries to do :history without :trace

[Pepe Iborra]

Sat Dec  8 10:09:18 PST 2007  Pepe Iborra <mnislaih at gmail.com>
  * Help the user when she tries to do :history without :trace
  
  Teach GHCi to show a "perhaps you forgot to use :trace?" when
  it finds that the user is trying to retrieve an empty :history
  

    M ./compiler/ghci/InteractiveUI.hs -9 +13