Add accessor for Repo._initialized

I will need later on to verify whether a repository has been
initialized. Using the propery _initialized directly cause some linting
errors, so here is a trivial accessor around it.

Change-Id: I1b03031ff31e68115e9bea4e0bbcadf93772a386

zuul/merger/merger.py

@@ -58,6 +58,9 @@ class Repo(object):
         repo.config_writer().write()
         self._initialized = True
 
+    def isInitialized(self):
+        return self._initialized
+
     def createRepoObject(self):
         try:
             self._ensure_cloned()