I'm starting out with a file like this:
This is a
sentence broken
over many lines
without double spaces at
the ends of line. It
renders as a single
paragraph.
Then I run
pandoc -f markdown_github -t html -o markdown-linebreaks.html markdown-linebreaks.md
The resulting file is:
<p>This is a<br />
sentence broken<br />
over many lines<br />
without double spaces at<br />
the ends of line. It<br />
renders as a single<br />
paragraph.</p>
However, github runs the lines together, as html does, as long as none ends with two spaces.