Updated version of git-hunk-commit.bash

Hi,

on IRC, "cehteh" requested that I send an updated version of the script. 
It was submitted when a short-lived option of git-diff was used, so the 
first publicised version no longer works.

This has seen a tremendous amount of work by Jason Chu, which made it work 
more like darcs when called as "bash git-hunk-commit.bash --darcs".

It still relies heavily on bash arrays.

Ciao,
Dscho