Add support for item.change for pipeline start-message formater

By adding that attribute in the formater it is then possible to
report the buildset status URL as reporter start message.

For instance:

start-message: Build started. Ephemeral buildset status {status_url}
               /{pipeline.tenant.name}/status/change/{change.number},
               {change.patchset}.

Change-Id: I5f7503e4babc6b84b20292f2063ffd90cb6065d9

tests/fixtures/layouts/start-message.yaml

@@ -1,7 +1,7 @@
 - pipeline:
     name: gate
     manager: dependent
-    start-message: Jobs started for {pipeline.name}.
+    start-message: Jobs started in {pipeline.name} for {change.number},{change.patchset}.
     trigger:
       gerrit:
         - event: comment-added

tests/unit/test_scheduler.py

@@ -4827,10 +4827,7 @@ For CI problems and help debugging, contact ci@example.org"""
         self.waitUntilSettled()
 
         self.assertEqual(1, len(self.smtp_messages))
-
-        start_msg = """\
-Jobs started for gate."""
-
+        start_msg = "Jobs started in gate for 1,1."
         self.assertTrue(self.smtp_messages[0]['body'].startswith(start_msg))
 
     @simple_layout('layouts/unmanaged-project.yaml')

zuul/reporter/__init__.py

@@ -128,8 +128,13 @@ class BaseReporter(object, metaclass=abc.ABCMeta):
     def _formatItemReportStart(self, item, with_jobs=True):
         status_url = get_default(self.connection.sched.config,
                                  'web', 'status_url', '')
-        return item.pipeline.start_message.format(pipeline=item.pipeline,
-                                                  status_url=status_url)
+        if status_url:
+            status_url = item.formatUrlPattern(status_url)
+
+        return item.pipeline.start_message.format(
+            pipeline=item.pipeline.getSafeAttributes(),
+            change=item.change.getSafeAttributes(),
+            status_url=status_url)
 
     def _formatItemReportSuccess(self, item, with_jobs=True):
         msg = item.pipeline.success_message