To facilitate merge handling, add a node_absent() callback to the diff tree processor.

* subversion/include/private/svn_diff_tree.h
  (svn_diff_tree_processor_t): Add several comments and node_absent() callback.

* subversion/libsvn_diff/diff_tree.c
  (default_node_absent): New function.
  (svn_diff__tree_processor_create): Hook node_absent.

  (reverse_node_absent): New function.
  (svn_diff__tree_processor_reverse_create): Hook node_absent.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1438980 13f79535-47bb-0310-9956-ffa450edef68