Move deprecated svn_diff_mem_string_output_merge() into the deprecated file.

* subversion/libsvn_diff/diff_memory.c
  (svn_diff_mem_string_output_merge): Remove from here...

* subversion/libsvn_diff/deprecated.c
  (svn_diff_mem_string_output_merge): and put it here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618888 13f79535-47bb-0310-9956-ffa450edef68