Git fetcher: Improve error message for untracked files (backport #12870)

[mergify]

Motivation

It now says:

error: Path 'foo/bar' in the repository "/path/to/repo" is not tracked by Git.

       To make it visible to Nix, run:

       git -C "/path/to/repo" add "foo/bar"

Context

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

---

This is an automatic backport of pull request #12870 done by [Mergify](https://mergify.com).

[mergify]

This pull request has been removed from the queue for the following reason: pull request dequeued.

Pull request #12884 has been dequeued. The pull request could not be merged. This could be related to an activated branch protection or ruleset rule that prevents us from merging. (details: Required status check "tests on ubuntu" is in progress.)

You should look at the reason for the failure and decide if the pull request needs to be fixed or if you want to requeue it.
If you do update this pull request, it will automatically be requeued once the queue conditions match again.
If you think this was a flaky issue instead, you can requeue the pull request, without updating it, by posting a @mergifyio requeue comment.