Clean hierarchy of calls between models and Gitalb::Git for blob search

app/models/repository.rb

@@ -468,9 +468,7 @@ class Repository
   end
 
   def blob_at(sha, path)
-    unless Gitlab::Git.blank_ref?(sha)
-      Blob.decorate(Gitlab::Git::Blob.find(self, sha, path), project)
-    end
+    Blob.decorate(raw_repository.blob_at(sha, path), project)
   rescue Gitlab::Git::Repository::NoRepository
     nil
   end

lib/gitlab/git/repository.rb

@@ -1092,6 +1092,10 @@ module Gitlab
         popen(args, @path).last.zero?
       end
 
+      def blob_at(sha, path)
+        Gitlab::Git::Blob.find(self, sha, path) unless Gitlab::Git.blank_ref?(sha)
+      end
+
       def gitaly_repository
         Gitlab::GitalyClient::Util.repository(@storage, @relative_path, @gl_repository)
       end