Function staged_contents 

pub async fn staged_contents(
    root: &Path,
    wanted: fn(&Path) -> bool,
) -> Result<Vec<(PathBuf, Result<String>)>, GitError>

Each staged file wanted accepts, with the content the commit records for it.

The content comes from the same index as the list, never from the working tree, which differs from it after a partial git add and under an index the committer chose. A file whose content cannot be read carries its error, as reading it from disk would.