Add option for git configuration

[roberth]

Is your feature request related to a problem?

• Nix calls git
• git can have configuration specified on the command line

So Nix should forward such options

Proposed solution

Add a Nix (nix.conf) setting.
Those automatically become flags too.

Alternative solutions

Just a flag.

Additional context

Inventing a custom syntax for this would be un-fun for everyone involved.
Use semi-structured data

• nix.conf.json #8373

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

---

Add 👍 to issues you find important.