t/convert-compact: allow S_ISGID bit
    
    My user home directory on Alpine has S_ISGID set on it and every
    subdirectory inherits it.  This includes my work tree and the
    t/data-gen/* subdirectories.  So just ignore the presence (or
    non-presence) of the S_ISGID bit on directories descended from
    the cached t/data-gen/* directories.
    
    Now, public-inbox-convert may want to preserve S_ISGID on the
    newly-created v2 inbox, but that's a separate discussion.