Use gerrit.googlesource.com/git-repo as the default URL

This is basically the same repository, but may be slightly more
up-to-date than the one on code.google.com/p/git-repo.

Change-Id: I5c99539f53231958eefb6993f00997c9adf0a3c9

repo

@@ -2,7 +2,7 @@
 
 ## repo default configuration
 ##
-REPO_URL='https://code.google.com/p/git-repo/'
+REPO_URL='https://gerrit.googlesource.com/git-repo'
 REPO_REV='stable'
 
 # Copyright (C) 2008 Google Inc.