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

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

[mergify]

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