Fix a Mercurial crash with 'hg outgoing' and only warnings.
Review Request #4694 — Created Oct. 7, 2013 and updated
When trying to post without any outgoing changes, and with warnings
(for, say, unverified HTTPS certificates), things would crash. It
assumed if there was any output, there would be at least one non-warning
line, but this isn't always safe. We now check for this.