Only show changed URLs in the artist history, not unchanged URLs. Makes
the edit history more compact and easier to read by showing only the
things that changed on every edit.
- The types are:
-- Previous: The default and the previously used type
-- Subsequent: Compares against the next version
-- Current: Compares against the current version
- Allow switching between comparison types in index and diff views
-- Have links vary depending upon current comparison type
