Fix undefined constant Gitlab::Git::HooksService::PreReceiveError

Part of gitlab-org/gitlab-ce#49263

ee/app/services/projects/update_mirror_service.rb

@@ -49,7 +49,7 @@ module Projects
         else
           begin
             repository.ff_merge(current_user, upstream_branch.dereferenced_target, name)
-          rescue Gitlab::Git::HooksService::PreReceiveError, Gitlab::Git::CommitError => e
+          rescue Gitlab::Git::PreReceiveError, Gitlab::Git::CommitError => e
             errors << e.message
           end
         end

ee/spec/services/projects/update_mirror_service_spec.rb

@@ -25,6 +25,14 @@ describe Projects::UpdateMirrorService do
       described_class.new(project, project.owner).execute
     end
 
+    it 'rescues exceptions from Repository#ff_merge' do
+      stub_fetch_mirror(project)
+
+      expect(project.repository).to receive(:ff_merge).and_raise(Gitlab::Git::PreReceiveError)
+
+      expect { described_class.new(project, project.owner).execute }.not_to raise_error
+    end
+
     it "succeeds" do
       stub_fetch_mirror(project)