Only highlight search results under the highlighting size limit

We should use this limit everywhere, but especially in project search results,
where we could be highlighting very long single lines. (Typical examples:
minified JavaScript, and JSON data files.)

app/helpers/blob_helper.rb

 module BlobHelper
   def highlight(blob_name, blob_content, repository: nil, plain: false)
+    plain ||= blob_content.length > Blob::MAXIMUM_TEXT_HIGHLIGHT_SIZE
     highlighted = Gitlab::Highlight.highlight(blob_name, blob_content, plain: plain, repository: repository)
+
     raw %(<pre class="code highlight"><code>#{highlighted}</code></pre>)
   end
 

changelogs/unreleased/41666-cannot-search-with-keyword-merge-2.yml

+---
+title: Only highlight search results under the highlighting size limit
+merge_request: 16462
+author:
+type: performance

spec/helpers/blob_helper_spec.rb

@@ -22,6 +22,13 @@ describe BlobHelper do
       expect(result).to eq(%[<pre class="code highlight"><code><span id="LC1" class="line" lang="">:type "assem"))</span></code></pre>])
     end
 
+    it 'returns plaintext for long blobs' do
+      stub_const('Blob::MAXIMUM_TEXT_HIGHLIGHT_SIZE', 1)
+      result = helper.highlight(blob_name, blob_content)
+
+      expect(result).to eq(%[<pre class="code highlight"><code><span id="LC1" class="line" lang="">(make-pathname :defaults name</span>\n<span id="LC2" class="line" lang="">:type "assem"))</span></code></pre>])
+    end
+
     it 'highlights single block' do
       expected = %Q[<pre class="code highlight"><code><span id="LC1" class="line" lang="common_lisp"><span class="p">(</span><span class="nb">make-pathname</span> <span class="ss">:defaults</span> <span class="nv">name</span></span>
 <span id="LC2" class="line" lang="common_lisp"><span class="ss">:type</span> <span class="s">"assem"</span><span class="p">))</span></span></code></pre>]