Wikibase parsing of diff=... parameter was different from core

Core logik is basically:
* Check if the diff parameter does exists.
* If yes, check if it is equal to 'prev'.
* If it's equal to 'next'.
* Convert everything else to a number.
* If that conversion returns 0 compare to the current revision.
* Else try to use the number to find a revision.
* That may fail (e.g. for diff=-1).

In other words: In core every non-numeric value including 'cur',
'curr', 'c', '0' and even the empty string and null
(...?diff&oldid=123 will work) does exactly the same as 'cur'.

Bug: 61749
Change-Id: Id7aab6ec65a23128fffe4c9e366213603a7e59c6