sub diffpath_ix {
  my ($a, $b) = @_;
  my $last = 0;
  while ( $a=~m{(\G[^/]*/)}g ) {
    if ( substr($b, $last, length $1) eq $1 ) {
      $last = pos $a
    }
    else {
      return $last
    }
  }
  if (substr($a, $last) eq substr($b, $last)) {
    return length $a
  }
  return $last;
}