Use root ref instead of HEAD to find latest repository commit

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