Switching an FSFS txn internal function to the two-pool paradigm.

This is in preparation of a low-level API change.

* subversion/libsvn_fs_fs/transaction.c
  (rep_write_baton): Document what these pools are for by naming
                     them appropriately.
  (rep_write_cleanup,
   rep_write_get_baton): Update baton users.
  (get_shared_rep): Switch to two-pool paradigm and use the new
                    scratch pool wherever possible.
  (rep_write_contents_close): Update as caller and baton user.
  (write_container_rep,
   write_container_delta_rep): Update callers.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605068 13f79535-47bb-0310-9956-ffa450edef68