On the invoke-diff-cmd branch: Continuation from r1535411: Adjust
substitution syntax to %svn_(label)_[old|new|mine|yours|base].  Update
string assigment to use apr_pstrdup.

* subversion/libsvn_subr/io.c

  (__create_custom_diff_cmd): Update substitution syntax.  Update
    string assigment to use apr_pstrdup.

* subversion/svn/svn.c
  (svn_cl__options[], "invoke-diff-cmd"): Update help info.
  
* BRANCH-README: Update.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1537987 13f79535-47bb-0310-9956-ffa450edef68