Expire branch cache using correct method from GC

This used to call Repository#after_create_branch to expire the
cache. This has the side effect of tracking a `push_branch`
event. Which is not correct int he case of GC.

app/workers/git_garbage_collect_worker.rb

@@ -100,7 +100,7 @@ class GitGarbageCollectWorker # rubocop:disable Scalability/IdempotentWorker
   end
 
   def flush_ref_caches(project)
-    project.repository.after_create_branch
+    project.repository.expire_branches_cache
     project.repository.branch_names
     project.repository.has_visible_content?
   end

spec/workers/git_garbage_collect_worker_spec.rb

@@ -56,7 +56,7 @@ RSpec.describe GitGarbageCollectWorker do
 
       it "flushes ref caches when the task if 'gc'" do
         expect(subject).to receive(:renew_lease).with(lease_key, lease_uuid).and_call_original
-        expect_any_instance_of(Repository).to receive(:after_create_branch).and_call_original
+        expect_any_instance_of(Repository).to receive(:expire_branches_cache).and_call_original
         expect_any_instance_of(Repository).to receive(:branch_names).and_call_original
         expect_any_instance_of(Repository).to receive(:has_visible_content?).and_call_original
         expect_any_instance_of(Gitlab::Git::Repository).to receive(:has_visible_content?).and_call_original
@@ -77,7 +77,7 @@ RSpec.describe GitGarbageCollectWorker do
       end
 
       it 'returns silently' do
-        expect_any_instance_of(Repository).not_to receive(:after_create_branch).and_call_original
+        expect_any_instance_of(Repository).not_to receive(:expire_branches_cache).and_call_original
         expect_any_instance_of(Repository).not_to receive(:branch_names).and_call_original
         expect_any_instance_of(Repository).not_to receive(:has_visible_content?).and_call_original
 
@@ -102,7 +102,7 @@ RSpec.describe GitGarbageCollectWorker do
 
         it "flushes ref caches when the task if 'gc'" do
           expect(subject).to receive(:get_lease_uuid).with("git_gc:#{task}:#{project.id}").and_return(false)
-          expect_any_instance_of(Repository).to receive(:after_create_branch).and_call_original
+          expect_any_instance_of(Repository).to receive(:expire_branches_cache).and_call_original
           expect_any_instance_of(Repository).to receive(:branch_names).and_call_original
           expect_any_instance_of(Repository).to receive(:has_visible_content?).and_call_original
           expect_any_instance_of(Gitlab::Git::Repository).to receive(:has_visible_content?).and_call_original
@@ -170,7 +170,7 @@ RSpec.describe GitGarbageCollectWorker do
 
         it 'returns silently' do
           expect(subject).not_to receive(:command)
-          expect_any_instance_of(Repository).not_to receive(:after_create_branch).and_call_original
+          expect_any_instance_of(Repository).not_to receive(:expire_branches_cache).and_call_original
           expect_any_instance_of(Repository).not_to receive(:branch_names).and_call_original
           expect_any_instance_of(Repository).not_to receive(:has_visible_content?).and_call_original