Add access to Blob's language from gitattributes

Ported from Highlight class since it as a concept is more related to
blob, and this allows more flexibility.

app/models/blob.rb

@@ -184,6 +184,13 @@ class Blob < SimpleDelegator
     Gitlab::FileDetector.type_of(path) || Gitlab::FileDetector.type_of(name)
   end
 
+  def language_from_gitattributes
+    return nil unless project
+
+    repository = project.repository
+    repository.gitattribute(path, 'gitlab-language')
+  end
+
   def video?
     UploaderHelper::VIDEO_EXT.include?(extension)
   end

lib/gitlab/search_results.rb

@@ -25,6 +25,12 @@ module Gitlab
       def no_highlighting?
         false
       end
+
+      # Since search results often contain many items,
+      # not triggering lookup can avoid n+1 queries.
+      def language_from_gitattributes
+        nil
+      end
     end
 
     attr_reader :current_user, :query, :per_page

spec/models/blob_spec.rb

@@ -224,6 +224,22 @@ describe Blob do
     end
   end
 
+  describe '#language_from_gitattributes' do
+    subject(:blob) { fake_blob(path: 'file.md') }
+
+    it 'returns return value from gitattribute' do
+      expect(blob.project.repository).to receive(:gitattribute).with(blob.path, 'gitlab-language').and_return('erb?parent=json')
+
+      expect(blob.language_from_gitattributes).to eq('erb?parent=json')
+    end
+
+    it 'returns nil if project is absent' do
+      allow(blob).to receive(:project).and_return(nil)
+
+      expect(blob.language_from_gitattributes).to eq(nil)
+    end
+  end
+
   describe '#simple_viewer' do
     context 'when the blob is empty' do
       it 'returns an empty viewer' do