Consider this input:
This is preformatted:
line 1
line 2
line 3 after blank line
Before, this would split into two different <pre> sections
in Markdown mode. Now it matches legacy mode and doesn't.
Fixesgolang/go#37972.
Change-Id: I6bf156c76ef9c63b6c3ffe6cce431d9589def867
Reviewed-on: https://go-review.googlesource.com/c/tools/+/224958
Reviewed-by: Andrew Bonventre <andybons@golang.org>
