Patch cmark to allow nested documents

Patch taken from commonmark/cmark#522

dep-patches/cmark-allow-nested-documents.diff

@@ -0,0 +1,27 @@
+diff --git a/api_test/main.c b/api_test/main.c
+index c2db737..9bc5748 100644
+--- a/api_test/main.c
++++ b/api_test/main.c
+@@ -456,6 +456,7 @@ void hierarchy(test_batch_runner *runner) {
+
+   int list_item_flag = 1 << CMARK_NODE_ITEM;
+   int top_level_blocks =
++      (1 << CMARK_NODE_DOCUMENT) |
+       (1 << CMARK_NODE_BLOCK_QUOTE) | (1 << CMARK_NODE_LIST) |
+       (1 << CMARK_NODE_CODE_BLOCK) | (1 << CMARK_NODE_HTML_BLOCK) |
+       (1 << CMARK_NODE_PARAGRAPH) | (1 << CMARK_NODE_HEADING) |
+diff --git a/src/node.c b/src/node.c
+index c90b0d4..64a4ed7 100644
+--- a/src/node.c
++++ b/src/node.c
+@@ -39,10 +39,6 @@ static bool S_can_contain(cmark_node *node, cmark_node *child) {
+     }
+   }
+
+-  if (child->type == CMARK_NODE_DOCUMENT) {
+-    return false;
+-  }
+-
+   switch (node->type) {
+   case CMARK_NODE_DOCUMENT:
+   case CMARK_NODE_BLOCK_QUOTE:

flake.nix

@@ -170,6 +170,13 @@
             ];
           });
 
+          cmark-nix = final.cmark.overrideAttrs (o: {
+            patches = (o.patches or []) ++ [
+              # https://github.com/commonmark/cmark/pull/522
+              ./dep-patches/cmark-allow-nested-documents.diff
+            ];
+          });
+
           changelog-d-nix = final.buildPackages.callPackage ./misc/changelog-d.nix { };
 
           nix =
@@ -188,6 +195,7 @@
                 ;
               officialRelease = false;
               boehmgc = final.boehmgc-nix;
+              cmark = final.cmark-nix;
               libgit2 = final.libgit2-nix;
               busybox-sandbox-shell = final.busybox-sandbox-shell or final.default-busybox-sandbox-shell;
               changelog-d = final.changelog-d-nix;