Optimize MergeRequestDiff#last_commit_sha

The previous implementation would load all the MergeRequestDiffCommit
entries into memory and retrieve the SHA from the latest entry. We can
save a bit of database query time and memory by using the database to
retrieve the first item from the database.

Seen while investigating Todo API performance.
(https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25711)

app/models/merge_request_diff.rb

@@ -118,7 +118,7 @@ def commits
   end
 
   def last_commit_sha
-    commit_shas.first
+    merge_request_diff_commits.first&.sha
   end
 
   def first_commit

spec/models/merge_request_diff_spec.rb

 require 'spec_helper'
 
 describe MergeRequestDiff do
-  let(:diff_with_commits) { create(:merge_request).merge_request_diff }
+  let(:merge_request) { create(:merge_request) }
+  let(:diff_with_commits) { merge_request.merge_request_diff }
 
   describe 'create new record' do
     subject { diff_with_commits }
@@ -224,6 +225,18 @@
     end
   end
 
+  describe '#last_commit_sha' do
+    let(:commit_shas) { diff_with_commits.commit_shas }
+    let(:sorted_commits) { merge_request.merge_request_diff.merge_request_diff_commits.sort_by { |x| x.committed_date } }
+    let(:last_commit_sha) { diff_with_commits.last_commit_sha }
+
+    it 'returns the most recent commit SHA' do
+      expect(last_commit_sha).to eq(commit_shas.first)
+      # Sanity check the sort order
+      expect(last_commit_sha).to eq(sorted_commits.last.sha)
+    end
+  end
+
   describe '#commits_by_shas' do
     let(:commit_shas) { diff_with_commits.commit_shas }