Fix a Mercurial crash with 'hg outgoing' and only warnings.
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.

Ran 'rbt post' in a newly checked out repo, cloned from an HTTPS repo.

Instead of crashing with a ValueError, I got a proper error saying that

there weren't any modified files.