On the invoke-diff-cmd branch: Update parameter.

* subversion/svn/log-cmd.c
  (svn_client_diff_peg7): Change 'NULL' entry to pass invoke_diff_cmd.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1484733 13f79535-47bb-0310-9956-ffa450edef68