Allow --shift-heading-level-by=-1 to work in djot...

...in the same way it works for other formats (with the top-level
heading being promoted to metadata title).  This needed special
treatment because of the way djot surrounds sections with Divs.

Closes #10459.

src/Text/Pandoc/Transforms.hs

@@ -154,6 +154,11 @@ headerShift n (Pandoc meta (Header m _ ils : bs))
   | n < 0
   , m + n == 0 = headerShift n $
                  B.setTitle (B.fromList ils) $ Pandoc meta bs
+-- for this case, see #10459:
+headerShift n (Pandoc meta (Div attr@(_,"section":_,_) (Header m _ ils : as) : bs))
+  | n < 0
+  , m + n == 0 = headerShift n $
+                 B.setTitle (B.fromList ils) $ Pandoc meta (Div attr as : bs)
 headerShift n (Pandoc meta bs) = Pandoc meta (walk shift bs)
 
  where

test/command/10459.md

@@ -0,0 +1,11 @@
+```
+% pandoc -s --shift-heading-level-by=-1 -f djot -t native
+# hi
+^D
+Pandoc
+  Meta
+    { unMeta =
+        fromList [ ( "title" , MetaInlines [ Str "hi" ] ) ]
+    }
+  [ Div ( "hi" , [ "section" ] , [] ) [] ]
+```