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