On the invoke-diff-cmd branch: Update BRANCH-README file.  Update
'diff' help.

* BRANCH-README:
  Update to reflect current state.
  (svn_io_create_custom_diff_cmd): Remove.  
    
* subversion/svn/svn.c
  (svn_cl__options): Update help info for invoke-diff-cmd,


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1531535 13f79535-47bb-0310-9956-ffa450edef68