from the revision boxes. An analysis is now done whether the source and
  destinations should be on the left or right side.
- Fix the left_right case for merge lines to display correctly.
- Fix a +/-1 error on the merge lines to account for both rounding errors and
  the shadow of the revision boxes.
- Add configuration option 'merge_on_tag' to force the left_right case to
  display merge lines on the tags instead of on the top/bottom sides. This
  also solves imagemap overlaps where multiple sources or destinations would
  be displayed at the same position.
- Fix the imagemap function to record the correct position of the merges.