On the fsx-id branch:  Begin using our own dirent type within FSX.

There a number of reasons to do this:

- Decouple the implementation from the interface definition.
- Allow for getting rid of the svn_fs_id_t member within the struct
  while most times we would only need the dirents within FSX itself.
- Reduce the typical size of a dirent from 88 to 48 bytes under 64 bits.
  Compared to FSFS' 120 bytes, this grows RAM / cache capacity 2.5x.
- Eliminate ID construction and conversion overhead.

This patch will simply introduce dirent_t as a copy of svn_fs_dirent_t
and use it throughout FSX.  Convert at the interface.  Most of it is
one giant search & replace spree.

* subversion/libsvn_fs_x/fs.h
  (fs_x_data_t): Update commentary.
  (dirent_t): New struct, copied from svn_fs_dirent_t.

* subversion/libsvn_fs_x/cached_data.h
  (svn_fs_x__find_dir_entry,
   svn_fs_x__rep_contents_dir_entry): s/svn_fs_dirent_t/dirent_t/g

* subversion/libsvn_fs_x/cached_data.c
  (sorted,
   compare_dirent_name,
   read_dir_entries,
   get_dir_contents,
   svn_fs_x__find_dir_entry,
   svn_fs_x__rep_contents_dir_entry): Same.

* subversion/libsvn_fs_x/temp_serializer.h
  (svn_fs_x__extract_dir_entry,
   replace_baton_t,
   svn_fs_x__replace_dir_entry): Same.

* subversion/libsvn_fs_x/temp_serializer.c
  (dir_data_t,
   serialize_dir_entry,
   serialize_dir,
   deserialize_dir,
   find_entry,
   svn_fs_x__extract_dir_entry,
   slowly_replace_dir_entry,
   svn_fs_x__replace_dir_entry): Same.

* subversion/libsvn_fs_x/dag.h
  (svn_fs_x__dag_dir_entries,
   svn_fs_x__dag_dir_entry): Same.

* subversion/libsvn_fs_x/dag.c
  (dir_entry_id_from_node,
   svn_fs_x__dag_dir_entry,
   svn_fs_x__dag_delete,
   svn_fs_x__dag_delete_if_mutable): Same.

* subversion/libsvn_fs_x/transaction.c
  (unparse_dir_entry,
   unparse_dir_entries, 
   svn_fs_x__set_entry,
   write_directory_to_stream,
   write_final_rev): Same.

* subversion/libsvn_fs_x/tree.c
  (compare_dir_structure,
   merge): Same.
  (x_dir_entries): Convert from the internal dirent type to the FS API type.
  (crawl_directory_dag_for_mergeinfo,
   verify_node): s/svn_fs_dirent_t/dirent_t/g

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1647605 13f79535-47bb-0310-9956-ffa450edef68