Support dst-prefix and src-prefix in the .gitconfig file

Hello,

Recently Peter brought the `src-prefix = ./` and `dst-prefix = ./` as a 
possible solution to easy copy-paste of file location without needing to 
remove the prefix "[ab]/".
The problem lies in not being able to define these options in .gitconfig

I would love to have the option to do

[diff]
   src-prefix = ./
   dst-prefix = ./

It seems to be a much better solution than using the available `noprefix 
= true` which breaks multiple workflows.

David

-- 
David Heidelberg