Code simplification. No functional change.

* subversion/libsvn_fs_fs/tree.c
  (history_prev_args): Remove.
  (history_prev): Take the arguments as parameters rather than in a baton.
    Rename the two pools to our usual 'result_pool' and 'scratch_pool'.
  (fs_history_prev): Simplify the caller accordingly.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1484937 13f79535-47bb-0310-9956-ffa450edef68