Fix extract-changes.hs to use new changelog.md format.

tools/extract-changes.hs

@@ -7,6 +7,9 @@ import Text.Pandoc.JSON
 main = toJSONFilter extractFirst
 
 extractFirst :: Pandoc -> Pandoc
-extractFirst (Pandoc meta (Para{} : BulletList bs : _)) =
-  Pandoc meta [BulletList bs]
-extractFirst x = x
+extractFirst (Pandoc meta bs) =
+  let bs' = dropWhile (not . isSubhead) bs
+   in Pandoc meta (takeWhile (not . isSubhead) (drop 1 bs'))
+
+isSubhead (Header 2 _ _) = True
+isSubhead _ = False