Re-enable allowing n+1 Gitaly calls for cold cache

Whenever cache is enabled and cold, branches index still makes
n+1 calls. Make sure we catch this.

app/controllers/projects/branches_controller.rb

@@ -22,10 +22,14 @@ class Projects::BranchesController < Projects::ApplicationController
 
         @refs_pipelines = @project.pipelines.latest_successful_for_refs(@branches.map(&:name))
         @merged_branch_names = repository.merged_branch_names(@branches.map(&:name))
+
+        # n+1: https://gitlab.com/gitlab-org/gitaly/issues/992
+        Gitlab::GitalyClient.allow_n_plus_1_calls do
           @max_commits = @branches.reduce(0) do |memo, branch|
             diverging_commit_counts = repository.diverging_commit_counts(branch)
             [memo, diverging_commit_counts[:behind], diverging_commit_counts[:ahead]].max
           end
+        end
 
         render
       end

spec/controllers/projects/branches_controller_spec.rb

@@ -398,6 +398,22 @@ describe Projects::BranchesController do
       end
     end
 
+    # We need :request_store because Gitaly only counts the queries whenever
+    # `RequestStore.active?` in GitalyClient.enforce_gitaly_request_limits
+    # And the main goal of this test is making sure TooManyInvocationsError
+    # was not raised whenever the cache is enabled yet cold.
+    context 'when cache is enabled yet cold', :request_store do
+      it 'return with a status 200' do
+        get :index,
+            namespace_id: project.namespace,
+            project_id: project,
+            state: 'all',
+            format: :html
+
+        expect(response).to have_gitlab_http_status(200)
+      end
+    end
+
     context 'when branch contains an invalid UTF-8 sequence' do
       before do
         project.repository.create_branch("wrong-\xE5-utf8-sequence")
@@ -414,7 +430,7 @@ describe Projects::BranchesController do
       end
     end
 
-    context 'when depreated sort/search/page parameters are specified' do
+    context 'when deprecated sort/search/page parameters are specified' do
       it 'returns with a status 301 when sort specified' do
         get :index,
             namespace_id: project.namespace,