Do not call merger:cat when all config items are excluded

This patch aims to improve zuul startup by removing
call to merger:cat on projects that exclude all configuration
items (in the tenant configuration).

Change-Id: I3a48caa3ce78ff45ed0e12c3386f80357e9bab06
diff --git a/tests/unit/test_configloader.py b/tests/unit/test_configloader.py
index f7d580c..49220f2 100644
--- a/tests/unit/test_configloader.py
+++ b/tests/unit/test_configloader.py
@@ -213,6 +213,33 @@
                         project2_config.pipelines['check'].job_list.jobs)
 
 
+class TestTenantGroups4(TenantParserTestCase):
+    tenant_config_file = 'config/tenant-parser/groups4.yaml'
+
+    def test_tenant_groups(self):
+        tenant = self.sched.abide.tenants.get('tenant-one')
+        self.assertEqual(['common-config'],
+                         [x.name for x in tenant.config_projects])
+        self.assertEqual(['org/project1', 'org/project2'],
+                         [x.name for x in tenant.untrusted_projects])
+        project = tenant.config_projects[0]
+        tpc = tenant.project_configs[project.canonical_name]
+        self.assertEqual(self.CONFIG_SET, tpc.load_classes)
+        project = tenant.untrusted_projects[0]
+        tpc = tenant.project_configs[project.canonical_name]
+        self.assertEqual(set([]),
+                         tpc.load_classes)
+        project = tenant.untrusted_projects[1]
+        tpc = tenant.project_configs[project.canonical_name]
+        self.assertEqual(set([]),
+                         tpc.load_classes)
+        # Check that only one merger:cat job was requested
+        # org/project1 and org/project2 have an empty load_classes
+        cat_jobs = [job for job in self.gearman_server.jobs_history
+                    if job.name == b'merger:cat']
+        self.assertEqual(1, len(cat_jobs))
+
+
 class TestTenantUnprotectedBranches(TenantParserTestCase):
     tenant_config_file = 'config/tenant-parser/unprotected-branches.yaml'