Fix a pool usage issue with FSFS svn_fs_history_prev.  This may fix
a memory usage issue caused by cached DAG nodes not being released
in a timely manner. 

* subversion/libsvn_fs_fs/tree.c
  (fs_history_prev): Iterations shall use iterpools for scratch.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1647905 13f79535-47bb-0310-9956-ffa450edef68