Switch svn_fs_fs__rep_contents_dir to the two-pool paradigm.

* subversion/libsvn_fs_fs/cached_data.h
  (svn_fs_fs__rep_contents_dir): Replace POOL with RESULT_POOL plus
                                 SCRATCH_POOL and update the docstring.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__rep_contents_dir): Update implementation signature and
                                 use the scratch pool wherever feasible.
  (svn_fs_fs__rep_contents_dir_entry): Update API caller.

* subversion/libsvn_fs_fs/dag.c
  (svn_fs_fs__dag_dir_entries): Ditto.

* subversion/libsvn_fs_fs/transaction.c
  (svn_fs_fs__set_entry,
   write_final_rev): Ditto.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554628 13f79535-47bb-0310-9956-ffa450edef68