Org: make code line comma-escaping match Emacs' behavior.

Org's escaping rule for code in src/example blocks adds a comma to
lines matching `^[ \t]*,*(\*|#\+)`, i.e. lines already starting with
commas before `*` or `#+` get an additional comma; unescaping removes
one comma from such lines. The writer previously left a literal
`,#+foo` line unescaped, and the reader then stripped its comma when
reading the result back, corrupting the code on round trips.

Writer and reader now both handle runs of commas, matching
org-escape-code-in-region and org-unescape-code-in-region.

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -274,11 +274,15 @@ rawBlockContent' blockEnder = try $ do
    rawLine :: Monad m => OrgParser m Text
    rawLine = try $ ("" <$ blankline) <|> anyLine
 
+   -- Remove one comma from lines consisting of any number of commas
+   -- followed by "*" or "#+", like Emacs' org-unescape-code-in-region.
    commaEscaped suff = case T.uncons suff of
      Just (',', cs)
-       | "*"  <- T.take 1 cs -> cs
-       | "#+" <- T.take 2 cs -> cs
-     _                       -> suff
+       | isEscaped cs -> cs
+     _                -> suff
+    where
+     isEscaped cs = let cs' = T.dropWhile (== ',') cs
+                    in "*" `T.isPrefixOf` cs' || "#+" `T.isPrefixOf` cs'
 
 -- | Read but ignore all remaining block headers.
 ignHeaders :: Monad m => OrgParser m ()

src/Text/Pandoc/Writers/Org.hs

@@ -213,13 +213,18 @@ blockToOrg (CodeBlock (ident,classes,kvs) str) = do
   let (beg, end) = case lang of
         Nothing -> ("#+begin_example" <> numberlines, "#+end_example")
         Just x  -> ("#+begin_src " <> x <> numberlines <> args, "#+end_src")
-  -- escape special lines
+  -- Escape special lines by prepending a comma. Like Emacs'
+  -- org-escape-code-in-region, escape all lines consisting of
+  -- indentation, then any number of commas, then "*" or "#+"; this
+  -- keeps lines that already start with commas intact when the block
+  -- is unescaped again.
+  let needsEscape t = let t' = T.dropWhile (== ',') t
+                      in T.isPrefixOf "#+" t' || T.isPrefixOf "*" t'
   let escape_line line =
         let (spaces, code) = T.span (\c -> c == ' ' || c == '\t') line
-        in spaces <>
-           (if T.isPrefixOf "#+" code || T.isPrefixOf "*" code
-            then T.cons ',' code
-            else code)
+        in if needsEscape code
+           then spaces <> T.cons ',' code
+           else line
   let escaped = T.unlines . map escape_line . T.lines $ str
   return $ name $$ literal beg $$ literal escaped $$ literal end $$ blankline
 blockToOrg (BlockQuote blocks) = do