Add a simple filter tree processor which can cut a relpath component
from the path passed to the callbacks.

Will be used by my next patch to remove yet another (new in 1.8) diff driver.

* subversion/include/private/svn_diff_tree.h
  (svn_diff__tree_processor_filter_create): New function.

* subversion/libsvn_diff/diff_tree.c
  (includes): Add assert.h.
  (reverse_node_absent): Fix processor reference.
  (filter_*): New wrappers.
  (svn_diff__tree_processor_filter_create): New function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1444209 13f79535-47bb-0310-9956-ffa450edef68