Stop parsing "list lines" when we hit a block tag.

This fixes exponential slowdown in certain input, e.g.
a series of lists followed by `</div>`.

src/Text/Pandoc/Readers/Markdown.hs

@@ -732,7 +732,8 @@ listLine = try $ do
   notFollowedBy' (do indentSpaces
                      many (spaceChar)
                      listStart)
-  chunks <- manyTill (liftM snd (htmlTag isCommentTag) <|> count 1 anyChar) newline
+  chunks <- manyTill (liftM snd (htmlTag isCommentTag) <|> count 1 (satisfy (/='<'))
+                    <|> (notFollowedBy' (htmlTag isBlockTag) >> count 1 anyChar)) newline
   return $ concat chunks
 
 -- parse raw text for one list item, excluding start marker and continuations
@@ -759,6 +760,7 @@ listContinuationLine :: MarkdownParser String
 listContinuationLine = try $ do
   notFollowedBy blankline
   notFollowedBy' listStart
+  notFollowedBy' $ try $ skipMany spaceChar >> htmlTag (~== TagClose "div")
   optional indentSpaces
   result <- anyLine
   return $ result ++ "\n"