Use svn_error_trace() in implementation of svn_diff_mem_string_output_merge2().

* subversion/libsvn_diff/deprecated.c
  (svn_diff_mem_string_output_merge2): Wrap in svn_error_trace().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618665 13f79535-47bb-0310-9956-ffa450edef68