Slightly improve status reporting.

When the metabook spec already has a commonswiki, we want to 'skip over'
a status report.  Passing no arguments made us repeat the last status report
instead, which made it look like we were downloading site information twice
for the last wiki.  Use a space instead.

Change-Id: I9f44371d090713fde3c529be027cac71432d4e6a