Merge branch 'fix/gb/make-root-ref-default-instead-of-head' into 'master'

Use root ref instead of HEAD to find latest repository commit

Closes gitlab-ee#5864

See merge request gitlab-org/gitlab-ce!20258

app/models/repository.rb

@@ -99,11 +99,11 @@ class Repository
     "#<#{self.class.name}:#{@disk_path}>"
   end
 
-  def commit(ref = 'HEAD')
+  def commit(ref = nil)
     return nil unless exists?
     return ref if ref.is_a?(::Commit)
 
-    find_commit(ref)
+    find_commit(ref || root_ref)
   end
 
   # Finding a commit by the passed SHA

spec/models/repository_spec.rb

@@ -479,6 +479,14 @@ describe Repository do
       end
     end
 
+    context 'when ref is not specified' do
+      it 'is using a root ref' do
+        expect(repository).to receive(:find_commit).with('master')
+
+        repository.commit
+      end
+    end
+
     context 'when ref is not valid' do
       context 'when preceding tree element exists' do
         it 'returns nil' do