Markdown reader: Fix parsing bug with nested fenced divs.

Closes #4281.

Previously we allowed "nonindent spaces" before the
opening and closing `:::`, but this interfered with
list parsing, so now we require the fences to be
flush with the margin of the containing block.

src/Text/Pandoc/Readers/Markdown.hs

@@ -1971,7 +1971,6 @@ divHtml = try $ do
 divFenced :: PandocMonad m => MarkdownParser m (F Blocks)
 divFenced = try $ do
   guardEnabled Ext_fenced_divs
-  nonindentSpaces
   string ":::"
   skipMany (char ':')
   skipMany spaceChar
@@ -1986,7 +1985,6 @@ divFenced = try $ do
 
 divFenceEnd :: PandocMonad m => MarkdownParser m ()
 divFenceEnd = try $ do
-  nonindentSpaces
   string ":::"
   skipMany (char ':')
   blanklines

test/command/4281.md

@@ -0,0 +1,18 @@
+```
+% pandoc -t native
+:::: {.a}
+- ::: {.b}
+  text
+  :::
+  ::: {.c}
+  text
+  :::
+::::
+^D
+[Div ("",["a"],[])
+ [BulletList
+  [[Div ("",["b"],[])
+    [Para [Str "text"]]
+   ,Div ("",["c"],[])
+    [Para [Str "text"]]]]]]
+```