"Version $request.rev2" displayed in the version compare interface

When not selecting a "from" or "to" version and displaying the version compare, the code assumes the parameters are always there, and compares some null objects.

It should behave like this:

• If no "from" and "to" versions are chosen, make a diff between the last 2 versions
• If no "from" is chosen, compare between "to-1" to "to"
• In no "to" is chosen, compare between "from" and "from+1".

-1 and +1 should take into consideration the "show minor edits" option.