Fine-tune our FSFS format7 reordering strategy:
Cluster all directory nodes putting them in front of all file nodes.
Within these sections, apply the same heuristics as we did before.
This reduces the I/O during "loggy" operations and random DAG walks.
Checkouts are widely unaffected.

* subversion/libsvn_fs_fs/pack.c
  (path_order_t): Mark nodes as either being directory or file.
  (compare_path_order): Make node kind the first-level ordering criterion.
  (compare_is_dir): Yet another binary search predicate.
  (sort_reps_range): New sub-function factored out from ...
  (sort_reps): ... this one; call the range sort for both node types.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1559886 13f79535-47bb-0310-9956-ffa450edef68