The character ">" now only starts a blockquote if the resulting
blockquote would be non-empty. Thus, by itself, ">" is now
interpreted literally by bugdown, fixing #687. The message
with contents consisting of ">>>" is now parsed as a doubly
(not triply) nested blockquote with contents ">". Properly
formed blockquotes have identical behavior as before, but now
bugdown can no longer produce empty blockquotes as output.
Fixes#2886, #687.
83f563aeff