New Hook rc/watchlist hook ChangesListBegin

An extension hooking into recent changes may need to load additional
data that did not fit into the recentchanges table.  This hook gives
revisions an opportunity to see the full result prior to rendering
and batch load where approprite rather than loading piecemeal during
the render process.

Change-Id: I28d4e41437e485e518f2a23b6da00cdc430a8c23
Merge "New Hook rc/watchlist hook ChangesListBegin"