Commonmark writer: fix regression with fenced divs.

Starting with 2.10.1, fenced divs no longer render with
HTML div tags in commonmark output.  This is a regression
due to our transition from cmark-gfm.  This commit fixes it.

Closes #6768.

src/Text/Pandoc/Writers/Markdown.hs

@@ -419,6 +419,7 @@ blockToMarkdown' :: PandocMonad m
 blockToMarkdown' _ Null = return empty
 blockToMarkdown' opts (Div attrs ils) = do
   contents <- blockListToMarkdown opts ils
+  variant <- asks envVariant
   return $
     case () of
          _ | isEnabled Ext_fenced_divs opts &&
@@ -428,7 +429,8 @@ blockToMarkdown' opts (Div attrs ils) = do
                 literal ":::" <> blankline
            | isEnabled Ext_native_divs opts ||
              (isEnabled Ext_raw_html opts &&
-              isEnabled Ext_markdown_in_html_blocks opts) ->
+              (variant == Commonmark ||
+               isEnabled Ext_markdown_in_html_blocks opts)) ->
                 tagWithAttrs "div" attrs <> blankline <>
                 contents <> blankline <> "</div>" <> blankline
            | isEnabled Ext_raw_html opts &&

test/command/6768.md

@@ -0,0 +1,21 @@
+```
+% pandoc -tcommonmark
+::: custom_div
+This is a div
+:::
+^D
+<div class="custom_div">
+
+This is a div
+
+</div>
+```
+
+```
+% pandoc -tcommonmark-raw_html
+::: custom_div
+This is a div
+:::
+^D
+This is a div
+```