FilePage.download(): add revision parameter to download arbitrary revision
FilePage.download(): add revision parameter to download arbitrary revision

The parameter will default to the latest revision when unprovided,
so that this method remain backwards-compatiable. The revision
(instance of FileInfo) will provide the url and sha1 for the download
and verification process.

Bug: T166939
Change-Id: I25842ae4276fbb45ba69a31d66b67401723e3ee5

happy5214/pywikibot-core,PersianWikipedia/pywikibot-core,npdoty/pywikibot,magul/pywikibot-core,magul/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core,hasteur/g13bot_tools_new,happy5214/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,hasteur/g13bot_tools_new