On the fsfs-format7 branch: fix a function used by our pack algo.

* subversion/libsvn_fs_fs/index.c
  (svn_fs_fs__l2p_get_max_ids): got difference backwards

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1445039 13f79535-47bb-0310-9956-ffa450edef68