Turns out, if you actually pass the environment, it doesn't like it because 0 isn't a string!

Author: gwynne

utils/update_checkout/update_checkout/update_checkout.py

@@ -261,7 +261,7 @@ def obtain_additional_swift_sources(pool_args):
      skip_history, skip_tags, skip_repository_list) = pool_args
 
     env = dict(os.environ)
-    env.update({'GIT_TERMINAL_PROMPT': 0})
+    env.update({'GIT_TERMINAL_PROMPT': '0'})
 
     with shell.pushd(args.source_root, dry_run=False, echo=False):