Flow uses a cross-wiki database (on the extension1 cluster). We need some way to point maintenance operations at it. Normally you indicate the database (and the version of MediaWiki) using the --wiki=flowdb option, but this fails because "flowdb" is not in wikiversions.cdb, and there are probably other assumptions that the wiki option is more than just database name. A separate --wikidb= option expresses the intent better.
Aaron already wrote the patch, gerrit 100707, we just need to backport it to wmf6 for Flow deployment.