GitHub has two Markdown modes, one for long-form documents like READMEs and one for short things like issue coments. In issue comments, a line break is treated as a hard line break. In README, wikis, etc., it is treated as a space as in regular Markdown. Since pandoc is more likely to be used to convert long-form documents from GitHub Markdown, `-hard_line_breaks` is a better default. Closes #3594.