Minor refactoring on Gitlab::Git

lib/gitlab/git.rb

 module Gitlab
   module Git
-    BLANK_SHA = '0' * 40
-    TAG_REF_PREFIX = "refs/tags/"
-    BRANCH_REF_PREFIX = "refs/heads/"
+    # '0' * 40 -- this was easyer to freeze
+    BLANK_SHA = "0000000000000000000000000000000000000000".freeze
+    TAG_REF_PREFIX = "refs/tags/".freeze
+    BRANCH_REF_PREFIX = "refs/heads/".freeze
 
     class << self
       def ref_name(ref)

spec/lib/gitlab/git_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Git do
+  describe "BLANK_SHA" do
+    it "is a string of 40 zero's" do
+      expect(Gitlab::Git::BLANK_SHA).to eq('0' * 40)
+    end
+  end
+end