workaround: restore CategoryMoveRobot.oldcat after move

pywikibot.data.api.update_page would otherwise make oldcat point to
newcat, making CategoryMoveRobot._change() almost useless.

I would surely welcome a more thorough solution.

Change-Id: I8842fbba10c9026a9087dfb1503708644f2992fd
Merge "workaround: restore CategoryMoveRobot.oldcat after move"