FIX: Internal oneboxes with github links (#22829)

Internal oneboxes to posts that contained oneboxed github links to
commits or PRs with long enough commit messages to have the `show-more`
and the `excerpt hidden` classes in their html were being stripped of
their content resulting in empty internal oneboxes.

see: https://meta.discourse.org/t/269436

This fixes a regression introduced in:

0b3cf83e3c7d03c2dcd54f86de60efe32f385a44

lib/excerpt_parser.rb

@@ -114,7 +114,11 @@ class ExcerptParser < Nokogiri::XML::SAX::Document
       @in_quote = !@keep_onebox_source if attributes.include?(%w[class source])
     when "div", "span"
       attributes = Hash[*attributes.flatten]
-      if attributes["class"]&.include?("excerpt")
+
+      # Only match "excerpt" class if it does specifically equal "excerpt
+      # hidden" in order to prevent internal links with GitHub oneboxes from
+      # being empty https://meta.discourse.org/t/269436
+      if attributes["class"]&.include?("excerpt") && !attributes["class"]&.match?("excerpt hidden")
         @excerpt = +""
         @current_length = 0
         @start_excerpt = true