Function write_sources 

pub async fn write_sources(
    uri: &str,
    files: &[SourceFile],
    merge: bool,
) -> Result<(usize, usize), Error>

Store files: contents go into source_blobs once per hash (append-only, so unchanged files cost nothing), and source_files maps each path to its hash. With merge, rows of repositories files does not cover are kept. Returns the files and the new contents written.