Merge branch 'jk/quarantine-received-objects'

Add finishing touches to a recent topic.

* jk/quarantine-received-objects:
  refs: reject ref updates while GIT_QUARANTINE_PATH is set
  receive-pack: document user-visible quarantine effects
  receive-pack: drop tmp_objdir_env from run_update_hook

refs.c

@@ -1643,6 +1643,12 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 {
 	struct ref_store *refs = transaction->ref_store;
 
+	if (getenv(GIT_QUARANTINE_ENVIRONMENT)) {
+		strbuf_addstr(err,
+			      _("ref updates forbidden inside quarantine environment"));
+		return -1;
+	}
+
 	return refs->be->transaction_commit(refs, transaction, err);
 }