enhancement of history loading/saving
enhancement of history loading/saving

Ignore-this: 80da3afd582f877a041bbc15a56b7a0a

When saving the history, it is merged with the on disk history.

darcs-hash:20100116191737-c41ad-a5c8a41f09f5f831af50c55a2ffa02a25a88f695

mirage/lwt,rgrinberg/lwt,ocsigen/lwt,fdopen/lwt,c-cube/lwt,mfp/lwt,rneswold/lwt,c-cube/lwt,janestreet/lwt-async,gfxmonk/lwt,rneswold/lwt,rgrinberg/lwt,janestreet/lwt-async,mirage/lwt,fdopen/lwt,ocsigen/lwt,rgrinberg/lwt,mfp/lwt,rneswold/lwt,rgrinberg/lwt,c-cube/lwt,gfxmonk/lwt,ocsigen/lwt