Maintenance.php defines a constant MW_DB based on the --wiki parameter, but at least according to Github's search, MW_DB is not used elsewhere.

I tried to run update.php using --wiki commonswiki (I confirmed that the database commonswiki existed):

vagrant@mediawiki-vagrant:/vagrant/mediawiki/maintenance$ php update.php --wiki commonswiki
MediaWiki 1.27alpha Updater

Your composer.lock file is up to date with current dependencies!
Going to run database updates for wiki

Its supposed to print out which wiki it's running database updates for, but it always seems to print the default, 'wiki'.