Eliminate unnecessary and duplicate system hook fires

Previously `SystemHookPushWorker` would always be called after a push event,
and this would queue a Sidekiq job regardless of whether any system hooks
needed that event. Moreover, another call inside `Project#execute_hooks` would
also fire system hooks if they existed.

This change both removes the duplicate system hook calls. For installations
without system hooks for push events, this change also can save significant
amount of RAM used by Redis.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50549

app/services/git_push_service.rb

@@ -140,7 +140,6 @@ class GitPushService < BaseService
     EventCreateService.new.push(project, current_user, build_push_data)
     Ci::CreatePipelineService.new(project, current_user, build_push_data).execute(:push)
 
-    SystemHookPushWorker.perform_async(build_push_data.dup, :push_hooks)
     project.execute_hooks(build_push_data.dup, :push_hooks)
     project.execute_services(build_push_data.dup, :push_hooks)
 

changelogs/unreleased/sh-conditional-system-hook-push.yml

@@ -0,0 +1,5 @@
+---
+title: Eliminate unnecessary and duplicate system hook fires
+merge_request: 21337
+author:
+type: performance

spec/services/git_push_service_spec.rb

@@ -223,9 +223,13 @@ describe GitPushService, services: true do
       end
     end
 
-    context "Sends System Push data" do
-      it "when pushing on a branch" do
-        expect(SystemHookPushWorker).to receive(:perform_async).with(push_data, :push_hooks)
+    describe 'system hooks' do
+      let(:system_hook_service) { double() }
+
+      it "sends a system hook after pushing a branch" do
+        expect(SystemHooksService).to receive(:new).and_return(system_hook_service)
+        expect(system_hook_service).to receive(:execute_hooks).with(push_data, :push_hooks)
+
         execute_service(project, user, oldrev, newrev, ref)
       end
     end