Git driver

This patch improves the existing git driver by adding
a refs watcher thread. This refs watcher looks at
refs added, deleted, updated and trigger a ref-updated
event.

When a refs is updated and that the related commits
from oldrev to newrev include a change on .zuul.yaml/zuul.yaml
or zuul.d/*.yaml then tenants including that ref is reconfigured.

Furthermore the patch includes a triggering model. Events are
sent to the scheduler so jobs can be attached to a pipeline for
running jobs.

Change-Id: I529660cb20d011f36814abe64f837945dd3f1f33

zuul/merger/client.py

@@ -131,6 +131,15 @@ class MergeClient(object):
         job = self.submitJob('merger:cat', data, None, precedence)
         return job
 
+    def getFilesChanges(self, connection_name, project_name, branch,
+                        tosha=None, precedence=zuul.model.PRECEDENCE_HIGH):
+        data = dict(connection=connection_name,
+                    project=project_name,
+                    branch=branch,
+                    tosha=tosha)
+        job = self.submitJob('merger:fileschanges', data, None, precedence)
+        return job
+
     def onBuildCompleted(self, job):
         data = getJobData(job)
         merged = data.get('merged', False)