Don't copy as GFM when more than GFM is selected

app/assets/javascripts/copy_as_gfm.js.es6

@@ -260,6 +260,9 @@
       const documentFragment = window.gl.utils.getSelectedFragment();
       if (!documentFragment) return;
+      // If the documentFragment contains more than just Markdown, don't copy as GFM.
+      if (documentFragment.querySelector('.md, .wiki')) return;
+
       e.preventDefault();
       clipboardData.setData('text/plain', documentFragment.textContent);

app/assets/javascripts/shortcuts_issuable.js

@@ -44,6 +44,9 @@
       documentFragment = window.gl.utils.getSelectedFragment();
       if (!documentFragment) return;
+      // If the documentFragment contains more than just Markdown, don't copy as GFM.
+      if (documentFragment.querySelector('.md, .wiki')) return;
+
       selected = window.gl.CopyAsGFM.nodeToGFM(documentFragment);
       replyField = $('.js-main-target-form #note_note');