Nix include git untracked files

[jfroche]

Patch Nix so that it doesn't ignore untracked files in git anymore and
continue to ignore files if they are in a .gitignore.

[soenkehahn]

I think we should have a test (maybe in GarnSpec.hs) that garn behaves this way now. Happy to pair on that.

[jfroche]

I have rebased the pull request in the Nix repository. Due to the recent changes in the main branch, specifically the transition to the libgit2 git implementation, the PR can no longer be directly applied with our current Nix version (2.17.1).
To address this, I've initiated a branch to backport the necessary changes for version 2.17.1.

[alexdavid]

This looks good to me. I think we can also remove the git add --intent-to-add flake.nix call in src/Garn/GarnConfig.hs:192 as well as the FAQ entry on the website at website/src/pages/main.tsx:263-281

[jfroche]

We still get the output of the git init during the test, with something like this:

    builds packages in a git repository with untracked files [ ]Initialized empty Git repository in /tmp/nix-shell.P8XVK8/mockery-5db85674c541bbc8/.git/
    builds packages in a git repository with untracked files [✔]

Not sure what would be the best to remove that.

I also see a lot of warning: Git tree '...' is dirty by nix during the tests, would it make sense to run nix with --no-warn-dirty to maybe reduce confusion for new comers ?