update submodules when doing git fetch

repo.py

@@ -98,6 +98,13 @@ def git_update(self):
 
             shutil.copy(f"git.{self.name}.out", self.suite.full_web_dir)
 
+        # update submodules to those specified by the current commit
+        # (--init is required because there may be new submodules since the last checkout)
+        _, _, rc = test_util.run(f"git submodule update --init")
+
+        if rc != 0:
+            self.suite.log.fail("ERROR: git submodule update was unsuccessful")
+
     def save_head(self):
         """Save the current head of the repo"""