Implement changes entry generation from SCM

Introduces three new service XML parameters:

  changesgenerate - Flag to trigger, defaults to "disable"
  changesrevision - Revision to diff against, defaults to HEAD - 10,
                    will be updated during successful run
  changesauthor   - Optional author of the changes entries, defaults to
                    first email= entry in ~/.oscrc or
                    "opensuse-packaging@opensuse.org"

All of these are optional and tar_scm will pick sane defaults or fail.
Changes entry generation is disabled by default but can be opted in by
adding <param name="changesgenerate">enable</param> to any _service file.

SCM agnostic but currently implemented only for the git SCM. Supports
multiple tar_scm services in one _service file. Uses lxml if available
to not destroy comments in the _service file (as the stdlib xml does).