[MERGE] Added support for a hook before sending the commit message to the editor.

[Ian Clatworthy]

Monty Taylor wrote:

>> For command line hooks, stdin and stdout could be used for existing and
>> new texts respectively.
> 
> Yes, ok. I've got a long plane flight tomorrow. I plan to work on this
> (and a few other things)

Cool.

Just to be clear, I'm working on enhancements to the general hook code
to support command line hooks - you don't need to do anything there,
just support the Python hook signature as discussed.

Ian C.