Asciidoctor: Handle snippets in definition lists (#680)

Our lang override processor had trouble with snippets inside of
definition lists because a method provided by Asciidoctor doesn't quite
behave well inside definition lists:
asciidoctor/asciidoctor#3133

That method had behavior that is a little too broad anyway so we
implement what we need directly.

Author: nik9000

resources/asciidoctor/lib/elastic_compat_tree_processor/extension.rb

@@ -67,7 +67,13 @@ def process_subs(block)
   }.freeze
 
   def process_lang_override(block)
-    next_block = block.next_adjacent_block
+    # Check if the next block is a marker for the language
+    # We don't want block.next_adjacent_block because that'll go "too far"
+    # and it has trouble with definition lists.
+    my_index = block.parent.blocks.find_index block
+    return unless my_index
+
+    next_block = block.parent.blocks[my_index + 1]
     return unless next_block && next_block.context == :paragraph
     return unless next_block.source =~ %r{pass:\[//\s*([^:\]]+)(?::\s*([^\]]+))?\]}
 

resources/asciidoctor/spec/elastic_compat_tree_processor_spec.rb

@@ -96,4 +96,27 @@
       expect(actual).to eq(expected.strip)
     end
   end
+
+  context 'a snippet is inside of a definition list' do
+    let(:converted) do
+      convert <<~ASCIIDOC
+        == Example
+        Term::
+        Definition
+        +
+        --
+        [source,js]
+        ----
+        GET /
+        ----
+        --
+      ASCIIDOC
+    end
+    let(:has_original_language) do
+      %r{<programlisting language="js" linenumbering="unnumbered">GET /</programlisting>}
+    end
+    it "doesn't break" do
+      expect(converted).to match(has_original_language)
+    end
+  end
 end