Extend merge request tests for all commits method

app/models/merge_request.rb

@@ -798,7 +798,8 @@ class MergeRequest < ActiveRecord::Base
     if persisted?
       merge_request_diffs.flat_map(&:commits_sha).uniq
     else
-      compare_commits.reverse.map(&:id)
+      cached_commits = compare_commits.to_a.reverse.map(&:id)
+      cached_commits.any? ? cached_commits : [diff_head_sha]
     end
   end
 

spec/models/merge_request_spec.rb

@@ -640,6 +640,7 @@ describe MergeRequest, models: true do
   end
 
   describe '#all_commits_sha' do
+    context 'when merge request is persisted' do
       let(:all_commits_sha) do
         subject.merge_request_diffs.flat_map(&:commits).map(&:sha).uniq
       end
@@ -669,6 +670,29 @@ describe MergeRequest, models: true do
       end
     end
 
+    context 'when merge request is not persisted' do
+      context 'when compare commits are set in the service' do
+        let(:commit) { spy('commit') }
+
+        subject do
+          build(:merge_request, compare_commits: [commit, commit])
+        end
+
+        it 'returns commits from compare commits temporary data' do
+          expect(subject.all_commits_sha).to eq [commit, commit]
+        end
+      end
+
+      context 'when compare commits are not set in the service' do
+        subject { build(:merge_request) }
+
+        it 'returns array with diff head sha element only' do
+          expect(subject.all_commits_sha).to eq [subject.diff_head_sha]
+        end
+      end
+    end
+  end
+
   describe '#participants' do
     let(:project) { create(:project, :public) }