Fix AttributeError when handle periodic job with github driver

Now, if we config periodic pipeline with github driver, the periodic jobs
won't be triggered and an AttributeError will be raised. That is because
the TimerTriggerEvent object don't have "commits" attribute which is used
in the zuul.driver.github.githubconnection.getPushedFileNames method.
We don't need the attribute assignment in periodic job, so this change
skipp it.

Change-Id: Ie1e85cc57ada17eacc44fb084f5f3f17aa8e9c45

tests/fixtures/layouts/timer-github.yaml

@@ -0,0 +1,25 @@
+- pipeline:
+    name: periodic
+    manager: independent
+    trigger:
+      timer:
+        - time: '* * * * * */1'
+
+- job:
+    name: base
+    parent: null
+    run: playbooks/base.yaml
+
+- job:
+    name: project-bitrot
+    nodeset:
+      nodes:
+        - name: static
+          label: ubuntu-xenial
+    run: playbooks/project-bitrot.yaml
+
+- project:
+    name: org/project
+    periodic:
+      jobs:
+        - project-bitrot

tests/unit/test_github_driver.py

@@ -210,6 +210,34 @@ class TestGithubDriver(ZuulTestCase):
         self.waitUntilSettled()
         self.assertEqual(1, len(self.history))
 
+    @simple_layout('layouts/basic-github.yaml', driver='github')
+    def test_timer_event(self):
+        self.executor_server.hold_jobs_in_build = True
+        self.commitConfigUpdate('org/common-config',
+                                'layouts/timer-github.yaml')
+        self.sched.reconfigure(self.config)
+        time.sleep(2)
+        self.waitUntilSettled()
+        self.assertEqual(len(self.builds), 1)
+        self.executor_server.hold_jobs_in_build = False
+        # Stop queuing timer triggered jobs so that the assertions
+        # below don't race against more jobs being queued.
+        self.commitConfigUpdate('org/common-config',
+                                'layouts/basic-github.yaml')
+        self.sched.reconfigure(self.config)
+        self.waitUntilSettled()
+        # If APScheduler is in mid-event when we remove the job, we
+        # can end up with one more event firing, so give it an extra
+        # second to settle.
+        time.sleep(1)
+        self.waitUntilSettled()
+        self.executor_server.release()
+        self.waitUntilSettled()
+        self.assertHistory([
+            dict(name='project-bitrot', result='SUCCESS',
+                 ref='refs/heads/master'),
+        ], ordered=False)
+
     @simple_layout('layouts/dequeue-github.yaml', driver='github')
     def test_dequeue_pull_synchronized(self):
         self.executor_server.hold_jobs_in_build = True

zuul/driver/github/githubconnection.py

@@ -721,7 +721,8 @@ class GithubConnection(BaseConnection):
             change.newrev = event.newrev
             change.url = self.getGitwebUrl(project, sha=event.newrev)
             change.source_event = event
-            change.files = self.getPushedFileNames(event)
+            if hasattr(event, 'commits'):
+                change.files = self.getPushedFileNames(event)
         return change
 
     def _getChange(self, project, number, patchset=None, refresh=False):