On the invoke-diff-cmd-feature branch: Add per file diff support which
uses an arbitrary config file.

* BRANCH-README: Update to reflect the latest changes.

* diff_cmd_config: Temporary file for demonstration/testing purposes.
    
* subversion/include/svn_io.h

  (svn_io_parse_diff_cmd_file): New temporary function.

* subversion/libsvn_subr/io.c

  (svn_io_run_external_diff): Add selection mechanism.

  (svn_io_parse_diff_cmd_file): New temporary function.

* subversion/svn/svn.c

  (svn_cl__options[]): Update svn diff help to explain the new
    feature.





git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1536020 13f79535-47bb-0310-9956-ffa450edef68