Function diff_asset 

pub fn diff_asset(
    asset_path: &AssetPath,
    project_dir: &Path,
    content_root: &Path,
) -> Result<Option<AssetDiff>, GitDiffError>

Compare the current on-disk version of asset_path against the HEAD-committed version. Returns None when the file is untracked, the repo has no prior commit, or the scan fails.