Implement changes entry generation from SCM

Introduces three 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"

All of these are optional and tar_scm will pick sane defaults or fail.
Changes entry generation is enabled by default but can be opted out by
adding <param name="changesgenerate">no</param> to any _service file.

SCM agnostic but currently implemented only for the git SCM. Supports
multiple tar_scm services in one _service file.