Update call to svn_client_diff_peg6 to svn_client_diff_peg7.

* subversion/svn/log-cmd.c 
  (svn_client_diff_peg7): Update call to svn_client_diff_peg6 to
    svn_client_diff_peg7.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1484251 13f79535-47bb-0310-9956-ffa450edef68