Use head tree (cached) for file search. Also add some tests

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/repository.rb

@@ -468,7 +468,11 @@ class Repository
   end
 
   def gitlab_ci_yml
-    @gitlab_ci_yml ||= blob_at(head_commit.sha, '.gitlab-ci.yml') unless empty?
+    return nil if empty?
+
+    @gitlab_ci_yml ||= tree(:head).blobs.find do |file|
+      file.name == '.gitlab-ci.yml'
+    end
   end
 
   def head_commit

spec/models/repository_spec.rb

@@ -148,6 +148,29 @@ describe Repository, models: true do
     end
   end
 
+  describe "#gitlab_ci_yml" do
+    before do
+      TestBlob = Struct.new(:name)
+    end
+
+    it 'returns valid file' do
+      files = [TestBlob.new('file'), TestBlob.new('.gitlab-ci.yml'), TestBlob.new('copying')]
+      expect(repository.tree).to receive(:blobs).and_return(files)
+
+      expect(repository.gitlab_ci_yml.name).to eq('.gitlab-ci.yml')
+    end
+
+    it 'returns nil if not exists' do
+      expect(repository.tree).to receive(:blobs).and_return([])
+      expect(repository.gitlab_ci_yml).to be_nil
+    end
+
+    it 'returns nil for empty repository' do
+      expect(repository).to receive(:empty?).and_return(true)
+      expect(repository.gitlab_ci_yml).to be_nil
+    end
+  end
+
   describe :add_branch do
     context 'when pre hooks were successful' do
       it 'should run without errors' do