`git_dir` parameter in zuul.conf sample

Change-Id: I95ef3dde919f4ab5ce471295fb7cc1568e42402a
Reviewed-on: https://review.openstack.org/13526
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
diff --git a/etc/zuul.conf-sample b/etc/zuul.conf-sample
index 41f2386..6c91c8f 100644
--- a/etc/zuul.conf-sample
+++ b/etc/zuul.conf-sample
@@ -13,3 +13,4 @@
 log_config=/etc/zuul/logging.yaml
 pidfile=/var/run/zuul/zuul.pid
 state_dir=/var/lib/zuul
+git_dir=/var/lib/zuul/git