CDRIVER-4422 calc_release_version.py: remove --merged, plus improvements (#1060)

Detail of changes:
 * remove --merged option for compatibility with older versions of 'git'
 * fix bug that considered pre-release tags as higher than their
   succeeding releases (e.g., 1.22.0-beta0 was treated as a higher
   version tag than 1.22.0)
 * refactor to reduce code duplication, increase readability, and
   improve maintainability

Co-authored-by: Kevin Albertson <kevin.albertson@10gen.com>