Don't do a commit lookup to present the MR entity, just return the sha

The MergeRequestWidgetEntity is used a lot in AJAX requests, and shaving
that git operation will hopefully translate in a performance improvement

app/serializers/merge_request_widget_entity.rb

@@ -38,7 +38,7 @@ class MergeRequestWidgetEntity < IssuableEntity
 
   # Diff sha's
   expose :diff_head_sha do |merge_request|
-    merge_request.diff_head_sha if merge_request.diff_head_commit
+    merge_request.diff_head_sha.presence
   end
 
   expose :merge_commit_message

spec/serializers/merge_request_widget_entity_spec.rb

@@ -147,9 +147,9 @@ describe MergeRequestWidgetEntity do
       allow(resource).to receive(:diff_head_sha) { 'sha' }
     end
 
-    context 'when no diff head commit' do
+    context 'when diff head commit is empty' do
       it 'returns nil' do
-        allow(resource).to receive(:diff_head_commit) { nil }
+        allow(resource).to receive(:diff_head_sha) { '' }
 
         expect(subject[:diff_head_sha]).to be_nil
       end
@@ -157,8 +157,6 @@ describe MergeRequestWidgetEntity do
 
     context 'when diff head commit present' do
       it 'returns diff head commit short id' do
-        allow(resource).to receive(:diff_head_commit) { double }
-
         expect(subject[:diff_head_sha]).to eq('sha')
       end
     end