Function set_projected 

fn set_projected(
    z: &mut ZoneTxn<'_>,
    heads: &mut Heads,
    dir_id: &str,
    file: &str,
    version: &str,
) -> Result<(), StoreError>

Point dir_id’s projection of file at version and log it (the caller writes heads.json, or it is written here when nothing else changes).