http://tehbemfqolualhxwkyimu3bdob5s4hkrfuwbwxzbblncjy2j2owgolqd.onion/documentation.html
The functions in this list should take two arguments, both of which are strings: the new directory, and the old directory. 'pre_command' is fired just before executing a subprocess-mode command. The functions in this list should take a single argument, which is a list representing the command that is to be run.