Clear merge requests counter cache after merge

Before this change, the MR counter in the sidebar would be wrong if an MR had
been merged since the last update, but not opened or closed, as merging did not
trigger a counter cache update.

app/services/merge_requests/post_merge_service.rb

@@ -14,6 +14,7 @@ module MergeRequests
       notification_service.merge_mr(merge_request, current_user)
       execute_hooks(merge_request, 'merge')
       invalidate_cache_counts(merge_request, users: merge_request.assignees)
+      merge_request.update_project_counter_caches
     end
 
     private

changelogs/unreleased/fix-mr-sidebar-counter-after-merge.yml

@@ -0,0 +1,5 @@
+---
+title: Fix merge request counter updates after merge
+merge_request:
+author:
+type: fixed

spec/services/merge_requests/post_merge_service_spec.rb

@@ -11,5 +11,16 @@ describe MergeRequests::PostMergeService do
 
   describe '#execute' do
     it_behaves_like 'cache counters invalidator'
+
+    it 'refreshes the number of open merge requests for a valid MR', :use_clean_rails_memory_store_caching do
+      # Cache the counter before the MR changed state.
+      project.open_merge_requests_count
+      merge_request.update!(state: 'merged')
+
+      service = described_class.new(project, user, {})
+
+      expect { service.execute(merge_request) }
+        .to change { project.open_merge_requests_count }.from(1).to(0)
+    end
   end
 end