Add param to allow scoped caching of Repo#merge_to_ref

Adding caching to this endpoint for all use cases triggered a bug in an
untested portion of Merge Trains. Adding this param will allow us to
selectively apply caching, improving the performance characteristics of
where we have a problem (MergeRequests::MergeabilityCheckService) but
not in the problematic Merge Trains code.

Changelog: added

app/services/merge_requests/merge_to_ref_service.rb

@@ -13,12 +13,12 @@ module MergeRequests
   class MergeToRefService < MergeRequests::MergeBaseService
     extend ::Gitlab::Utils::Override
 
-    def execute(merge_request)
+    def execute(merge_request, cache_merge_to_ref_calls = false)
       @merge_request = merge_request
 
       error_check!
 
-      commit_id = commit
+      commit_id = commit(cache_merge_to_ref_calls)
 
       raise_error('Conflicts detected during merge') unless commit_id
 
@@ -65,8 +65,9 @@ module MergeRequests
       params[:allow_conflicts] || false
     end
 
-    def commit
-      if Feature.enabled?(:cache_merge_to_ref_calls, project, default_enabled: false)
+    def commit(cache_merge_to_ref_calls = false)
+      if cache_merge_to_ref_calls &&
+        Feature.enabled?(:cache_merge_to_ref_calls, project, default_enabled: false)
         Rails.cache.fetch(cache_key, expires_in: 1.day) do
           extracted_merge_to_ref
         end

spec/services/merge_requests/merge_to_ref_service_spec.rb

@@ -43,28 +43,40 @@ RSpec.describe MergeRequests::MergeToRefService do
 
         # warm the cache
         #
-        service.execute(merge_request)
+        service.execute(merge_request, true)
       end
 
+      context 'when cache_merge_to_ref_calls parameter is true' do
         it 'caches the response', :request_store do
-        expect { 3.times { service.execute(merge_request) } }
+          expect { 3.times { service.execute(merge_request, true) } }
             .not_to change(Gitlab::GitalyClient, :get_request_count)
         end
       end
 
+      context 'when cache_merge_to_ref_calls parameter is false' do
+        it 'does not cache the response', :request_store do
+          expect(Gitlab::GitalyClient).to receive(:call).at_least(3).times.and_call_original
+
+          3.times { service.execute(merge_request, false) }
+        end
+      end
+    end
+
     context 'cache_merge_to_ref_calls flag disabled', :use_clean_rails_memory_store_caching do
       before do
         stub_feature_flags(cache_merge_to_ref_calls: false)
 
         # warm the cache
         #
-        service.execute(merge_request)
+        service.execute(merge_request, true)
       end
 
-      it 'does not cache the response', :request_store do
+      [true, false].each do |cache_merge_to_ref_calls|
+        it 'does not cache the response, regardless of cache_merge_to_ref_calls state', :request_store do
           expect(Gitlab::GitalyClient).to receive(:call).at_least(3).times.and_call_original
 
-        3.times { service.execute(merge_request) }
+          3.times { service.execute(merge_request, cache_merge_to_ref_calls) }
+        end
       end
     end
   end