Fix listing of untracked files to not include tracked but modified ones

Fix listing of untracked files to not include tracked but modified ones

The logic in `sources/path.rs` that finds files that are untracked by the
version control system was incorrectly including files that *are* tracked,
but had been modified without the modification being committed to Git.

This manifested itself as strange duplications of files (e.g. in `cargo
package --list`), since they were listed once as tracked files, and once
more as allegedly "untracked" ones. It also caused failures when trying to
package the crate when files had been deleted, but the deletion was not
yet committed to Git.

Fixes #2199