Allow previous state to be shown in state change log notes.

Allow previous state to be shown in state change log notes.

Wanrong Lin writes:

     My TODO sequence is set up as following:

     (setq org-todo-keywords '((sequence "TODO(t)" "STARTED(s!)"
     "WAITING(w@/!)"      "MAYBE(m/!)" "DELEGATED(g@/!)"
     "DEFERRED(d!/!)" "HOLD(h!/!)" "|" "DONE(x)"      "CANCELED(c)")))

     The state change logging is great, but I wonder whether we can
     further improve it:

     1. Can we also include the original state in the log message?
     i.e.: instead of having      - State "DEFERRED"   [2009-02-11 Wed 11:38]
     we can have
     - State "DEFERRED"  from "HOLD" [2009-02-11 Wed 11:38]

     The message will be clearer, and useful even when somebody edited
     the TODO keyword in place without using the "org-todo" command
     (in that case, simply looking at all the "destination states" in
     the log message does not give us the right information).

This strikes me like a good idea, so I have implemented it with this
commit.