loose: don't rewrite idx unnecessarily
loose: don't rewrite idx unnecessarily

As a minor optimization, don't rewrite the idx when there are no new
entries.

Signed-off-by: Ramkumar Ramachandra <5c355d1d38066ed0c5de420d67c1b5ed0687c4bd@gmail.com>

artagnon/phoenixfs,artagnon/phoenixfs,artagnon/phoenixfs