Show correct total number of commit diff's changes

Update the API endpoint to use the correct method for getting diff
of a commit to return correct X-Total in the response's header.

The said method uses the same limits as the ones used in the UI so
they'll be consistent.

changelogs/unreleased/7816-commits-diff-total-api.yml

+---
+title: Show correct total number of commit diff's changes
+merge_request: 19424
+author:
+type: fixed

lib/api/commits.rb

@@ -169,7 +169,7 @@ module API
 
         not_found! 'Commit' unless commit
 
-        raw_diffs = ::Kaminari.paginate_array(commit.raw_diffs.to_a)
+        raw_diffs = ::Kaminari.paginate_array(commit.diffs(expanded: true).diffs.to_a)
 
         present paginate(raw_diffs), with: Entities::Diff
       end

spec/requests/api/commits_spec.rb

@@ -1089,6 +1089,20 @@ describe API::Commits do
         expect(json_response.first.keys).to include 'diff'
       end
 
+      context 'when hard limits are lower than the number of files' do
+        before do
+          allow(Commit).to receive(:max_diff_options).and_return(max_files: 1)
+        end
+
+        it 'respects the limit' do
+          get api(route, current_user)
+
+          expect(response).to have_gitlab_http_status(200)
+          expect(response).to include_pagination_headers
+          expect(json_response.size).to be <= 1
+        end
+      end
+
       context 'when ref does not exist' do
         let(:commit_id) { 'unknown' }