On the fsfs-ucsnorm branch: Prevent modifications of nodes with paths
that contain name collisions.

* subversion/include/svn_error_codes.h
  (SVN_ERR_FS_NAME_COLLISION): New error code.
* subversion/libsvn_fs_fs/temp_serializer.c
  (get_entry_key): New; factored entry key deserialization from find_entry.
  (find_entry): Changed signature to return an svn_error_t*.
   Call get_entry_key instead of duplicating the deserialization in three
   (formerly two) places.
   If the entry was found, double-check that its key is unique.
  (svn_fs_fs__extract_dir_entry, svn_fs_fs__replace_dir_entry):
   Update calls to find_entry.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-ucsnorm@1563542 13f79535-47bb-0310-9956-ffa450edef68