workaround: restore CategoryMoveRobot.oldcat after move
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

Darkdadaah/pywikibot-core,jayvdb/pywikibot-core,emijrp/pywikibot-core,h4ck3rm1k3/pywikibot-core,jayvdb/pywikibot-core,magul/pywikibot-core,darthbhyrava/pywikibot-local,icyflame/batman,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,trishnaguha/pywikibot-core,valhallasw/pywikibot-core,PersianWikipedia/pywikibot-core,hasteur/g13bot_tools_new,smalyshev/pywikibot-core,magul/pywikibot-core,wikimedia/pywikibot-core,TridevGuha/pywikibot-core,npdoty/pywikibot,npdoty/pywikibot,hasteur/g13bot_tools_new,happy5214/pywikibot-core,h4ck3rm1k3/pywikibot-core,xZise/pywikibot-core,hasteur/g13bot_tools_new,VcamX/pywikibot-core,wikimedia/pywikibot-core