Fix inconsistent use of 'const' in the svn_diff_tree code.

Fixes GCC warning:
  subversion/libsvn_client/merge.c:9109: warning: passing argument 1 of
  'svn_wc__wrap_diff_callbacks' from incompatible pointer type

* subversion/include/private/svn_diff_tree.h.
  subversion/libsvn_diff/diff_tree.c
  (svn_diff__tree_processor_tee_create): Make both processor parameters 'const'.

* subversion/libsvn_client/merge.c
  (do_merge): Declare the callback processor as 'const'.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1440424 13f79535-47bb-0310-9956-ffa450edef68