On the invoke-diff-cmd-feature branch: Add invoke-diff-cmd help to svn
help log.

* subversion/svn/svn.c
  (svn_cl__cmd_table[]): Add opt_invoke_diff_cmd to svn_cl__log entry.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1535411 13f79535-47bb-0310-9956-ffa450edef68