Allow shas that are nil to compare to nil shas

Previously, nil == nil was being treated as false. This is both clearly
wrong and more seriously broke the equality contract of `DiffRefs`,
since the same object would not compare equal to itself it any of its
components was nil.

Adds a test for commit.diff_refs equality, the motivating case.

lib/gitlab/git.rb

@@ -88,6 +88,7 @@ module Gitlab
       end
 
       def shas_eql?(sha1, sha2)
+        return true if sha1.nil? && sha2.nil?
         return false if sha1.nil? || sha2.nil?
         return false unless sha1.class == sha2.class
 

spec/lib/gitlab/git_spec.rb

@@ -73,7 +73,8 @@ describe Gitlab::Git do
         [sha, short_sha,     true],
         [sha, sha.reverse,   false],
         [sha, too_short_sha, false],
-        [sha, nil,           false]
+        [sha, nil,           false],
+        [nil, nil,           true]
       ]
     end
 

spec/models/commit_spec.rb

@@ -63,6 +63,20 @@ describe Commit do
     end
   end
 
+  describe '#diff_refs' do
+    it 'is equal to itself' do
+      expect(commit.diff_refs).to eq(commit.diff_refs)
+    end
+
+    context 'from a factory' do
+      let(:commit) { create(:commit) }
+
+      it 'is equal to itself' do
+        expect(commit.diff_refs).to eq(commit.diff_refs)
+      end
+    end
+  end
+
   describe '#author', :request_store do
     it 'looks up the author in a case-insensitive way' do
       user = create(:user, email: commit.author_email.upcase)