On the fsfs-format7 branch:  Change the ordering strategy from rev-based
to name-based.  This maximises the chance of e.g. two files being close
in one pack file to also be close to one another in most other pack files.
So, optimized traversal along e.g. HEAD is also close to optimal along
the deltification chain.

Once at it, provide a different strategy for format6 and earlier repos.
See source comment for details.

* subversion/libsvn_fs_fs/pack.h
  (svn_fs_fs__order_dir_entries): we need an additional FS parameter

* subversion/libsvn_fs_fs/pack.c
  (compare_dir_entries): replaced by the following two
  (compare_dir_entries_format7): type, name-based ordering
  (compare_dir_entries_format6): rev, offset-based ordering
  (svn_fs_fs__order_dir_entries): use the appropriate ordering strategy
   based on the FS format
  (copy_node_to_temp): update caller

* subversion/libsvn_fs_fs/tree.c
  (fs_dir_optimal_order): update caller

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1446392 13f79535-47bb-0310-9956-ffa450edef68