For FSFS and FSX, fix a major merge scalability issue described here:
http://svn.haxx.se/dev/archive-2014-01/0057.shtml

The bottom line is that we allow incoming prop changes on directories
that have no addition nor deletion of entries relative to the TXN
base revision. This patch updates existing tests and provided a
specific new one.

* subversion/libsvn_fs_fs/tree.c
  (compare_dir_structure): A new utility function checking that all
                           dirents are still the same, although
                           possibly at different revisions.
  (merge): An dir prop change in a txn should not conflict with
           simply node upgrades within that directory.

* subversion/libsvn_fs_x/tree.c
  (compare_dir_structure, 
   merge): Similar implementation as for FSFS with a slight adaptation
           due to different directory container types.

* subversion/tests/libsvn_fs/fs-test.c
  (unordered_txn_dirprops): Update expected results.
  (dir_prop_merge): New test.
  (test_compat_version): Register the new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558224 13f79535-47bb-0310-9956-ffa450edef68