Update submodules if they're there

Some of our things, like new upstream gerrit, may have submodules
in them. In order to make sure that they, you know, work. We should
run some submodules update commands.

Change-Id: I7c47dd71a49108e79e27b5ba69fa09da638b3764

modules/jenkins/files/slave_scripts/gerrit-git-prep.sh

@@ -79,3 +79,10 @@ else
         git clean -x -f -d -q
     fi
 fi
+
+if [ -f .gitmodules ]
+then
+    git submodule init
+    git submodule sync
+    git submodule update --init
+fi