Freeze the expression instead of the literal

Also remove the spec for it

lib/gitlab/git.rb

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

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