Org reader: fix parsing of empty comment lines

Comment lines in Org-mode can be completely empty; both of these line
should produce no output:

    # a comment
    #

The reader used to produce a wrong result for the latter, but ignores
that line as well now.

Fixes: #5856

src/Text/Pandoc/Readers/Org/BlockStarts.hs

@@ -92,7 +92,9 @@ metaLineStart :: Monad m => OrgParser m ()
 metaLineStart = try $ skipSpaces <* string "#+"
 
 commentLineStart :: Monad m => OrgParser m ()
-commentLineStart = try $ skipSpaces <* string "# "
+commentLineStart = try $
+  -- the first char after '#' must be a plain space character or a newline
+  skipSpaces <* string "#" <* lookAhead (oneOf " \n")
 
 exampleLineStart :: Monad m => OrgParser m ()
 exampleLineStart = () <$ try (skipSpaces *> string ": ")

test/Tests/Readers/Org/Block.hs

@@ -73,13 +73,23 @@ tests =
       "----- em and en dash" =?>
       para "\8212\8211 em and en dash"
 
-  , "Comment Block" =:
+  , testGroup "Comments"
+    [ "Comment Block" =:
       T.unlines [ "#+BEGIN_COMMENT"
                 , "stuff"
                 , "bla"
                 , "#+END_COMMENT"] =?>
       (mempty::Blocks)
 
+    , "Comment line" =:
+      T.unlines [ "# this is a comment" ] =?>
+      (mempty :: Blocks)
+
+    , "Empty comment line" =:
+      T.unlines [ "  #" ] =?>
+      (mempty :: Blocks)
+    ]
+
   , testGroup "Blocks and fragments"
     [ "HTML block" =:
       T.unlines [ "#+BEGIN_HTML"