Fix cases where Markdown did not render links in activity feed

HTML would be stripped in `truncate_if_block` when a comment had multiple lines
and the first wasn't long enough to be truncated. The use of `node.content` would
strip all HTML tags. Using `node.inner_html` retains these tags and puts the
"..." in between paragraph tags.

Closes #2586

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 v 8.0.1
+  - Fix cases where Markdown did not render links in activity feed (Stan Hu)
   - Improve CI migration procedure and documentation
 v 8.0.0

app/helpers/gitlab_markdown_helper.rb

@@ -165,7 +165,7 @@ module GitlabMarkdownHelper
   # and return true.  Otherwise return false.
   def truncate_if_block(node, truncated)
     if node.element? && node.description.block? && !truncated
-      node.content = "#{node.content}..." if node.next_sibling
+      node.inner_html = "#{node.inner_html}..." if node.next_sibling
       true
     else
       truncated

spec/helpers/gitlab_markdown_helper_spec.rb

@@ -146,4 +146,24 @@ describe GitlabMarkdownHelper do
       expect(random_markdown_tip).to eq 'Random tip'
     end
   end
+
+  describe '#first_line_in_markdown' do
+    let(:text) { "@#{user.username}, can you look at this?\nHello world\n"}
+
+    it 'truncates Markdown properly' do
+      actual = first_line_in_markdown(text, 100, project: project)
+
+      doc = Nokogiri::HTML.parse(actual)
+
+      # Make sure we didn't create invalid markup
+      expect(doc.errors).to be_empty
+
+      # Leading user link
+      expect(doc.css('a').length).to eq(1)
+      expect(doc.css('a')[0].attr('href')).to eq user_path(user)
+      expect(doc.css('a')[0].text).to eq "@#{user.username}"
+
+      expect(doc.content).to eq "@#{user.username}, can you look at this?..."
+    end
+  end
 end