In FSFS, low-level internal API layer, rename pool arguments of single
pool functions in accordance to the two-pool paradigm.

No functional change.

* subversion/libsvn_fs_fs/low_level.h
  (svn_fs_fs__unparse_revision_trailer,
   svn_fs_fs__unparse_footer): These use a single RESULT_POOL.
  (svn_fs_fs__write_changes,
   svn_fs_fs__write_noderev,
   svn_fs_fs__write_rep_header): These use a single SCRATCH_POOL.

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__unparse_revision_trailer,
   svn_fs_fs__unparse_footer): These use a single RESULT_POOL.
  (svn_fs_fs__write_changes,
   svn_fs_fs__write_noderev,
   svn_fs_fs__write_rep_header): These use a single SCRATCH_POOL.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605060 13f79535-47bb-0310-9956-ffa450edef68