Use existing hash for cache key

app/models/compare.rb

@@ -26,7 +26,7 @@ class Compare
   end
 
   def cache_key
-    [@project, :compare, start_commit_sha, base_commit_sha, head_commit_sha, @straight]
+    [@project, :compare, diff_refs.hash]
   end
 
   def commits

spec/models/compare_spec.rb

@@ -19,9 +19,8 @@ RSpec.describe Compare do
     subject { compare.cache_key }
 
     it { is_expected.to include(project) }
-    it { is_expected.to include(start_commit.id) }
-    it { is_expected.to include(head_commit.id) }
-    it { is_expected.to include(compare.base_commit_sha) }
+    it { is_expected.to include(:compare) }
+    it { is_expected.to include(compare.diff_refs.hash) }
   end
 
   describe '#start_commit' do