Implement changes entry generation from SCM

Introduces new service XML parameters:

  changesgenerate - Flag to trigger, defaults to "enabled"
  changesrevision - Revision to diff MYREVISION against, defaults to
                    MYREVISION - 10, will be updated during successful
                    run
  changesauthor   - Optional author of the changes entries, defaults to
                    first email= in ~/.oscrc or "unknown"