make_spec.lua: fix migration of children nodes in create_anchors (#536)

* make_spec.lua: rename children to child

Naming a variable "children" when it holds one child is confusing, and blocks
naming actual children.

* make_spec.lua: fix migration of children nodes in create_anchors

Prior to this commit, when migrating the children of a node to a new one,
children are moved as they are iterated via `cmark_node_next`; however, when a
child is moved under the new node, its next relationship with its sibling is
broken, so in fact at most one child is migrated, and the rest are lost. This
resulted in cases like

    [Decimal numeric character
    references](@)
    consist of...

(note the softbreak) being eventually rendered as

    <a ...>Decimal numeric character</a>
    consist of...

tools/make_spec.lua

@@ -30,8 +30,8 @@ local extract_references = function(doc)
     if not entering and
         ((node_type == cmark.NODE_LINK and cmark.node_get_url(cur) == '@') or
           node_type == cmark.NODE_HEADING) then
-      local children = cmark.node_first_child(cur)
-      local label = trim(cmark.render_commonmark(children, OPT_DEFAULT, 0))
+      local child = cmark.node_first_child(cur)
+      local label = trim(cmark.render_commonmark(child, OPT_DEFAULT, 0))
       local ident = to_identifier(label)
       if refs[label] then
         warn("duplicate reference " .. label)
@@ -118,8 +118,8 @@ local create_anchors = function(doc, meta, to)
           node_type == cmark.NODE_HEADING) then
 
       local anchor
-      local children = cmark.node_first_child(cur)
-      local label = trim(cmark.render_commonmark(children, OPT_DEFAULT, 0))
+      local child = cmark.node_first_child(cur)
+      local label = trim(cmark.render_commonmark(child, OPT_DEFAULT, 0))
       local ident = to_identifier(label)
       if node_type == cmark.NODE_LINK then
         if format == 'latex' then
@@ -167,9 +167,13 @@ local create_anchors = function(doc, meta, to)
           end
         end
       end
-      while children do
-        node_append_child(anchor, children)
-        children = cmark.node_next(children)
+      local children = {};
+      while child do
+        children[#children + 1] = child
+        child = cmark.node_next(child)
+      end
+      for _,child in ipairs(children) do
+        node_append_child(anchor, child)
       end
       cmark.node_insert_before(cur, anchor)
       cmark.node_unlink(cur)
@@ -289,4 +293,3 @@ else
   io.stderr:write(msg)
   os.exit(1)
 end
-