Add user and previous rev to HistoryRevisionTools, and user to DiffRevisionTools

The next revision in the page history isn't necessarily the previous
revision (due to selective undeletions, history merges, etc). This
passes the next revision to HistoryRevisionTools so extensions can check
if needed. Also, it passes the user to this hook and DiffRevisionTools
to avoid use of wgUser or having to retrieve context.

Change-Id: Ibc68f19040eebe3614e07f753f26bbfd376ae28d