[git_cl] Apply OLD_AUTH if new auth flag is off

Note that this logic still isn't run yet, because we only call
ConfigureGitRepoAuth behind new auth flag checks.

Bug: b/351024645
Change-Id: Idca4d1ce3ebd3f2c673f244f85b3cae6da39f8e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5699188
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Allen Li <ayatane@chromium.org>

git_cl.py

@@ -3737,6 +3737,8 @@ class GitAuthConfigChanger(object):
     @staticmethod
     def _infer_mode() -> GitConfigMode:
         """Infer default mode to use."""
+        if not newauth.Enabled():
+            return GitConfigMode.OLD_AUTH
         if gerrit_util.ShouldUseSSO(gerrit_host):
             return GitConfigMode.NEW_AUTH_SSO
         return GitConfigMode.NEW_AUTH