[git_cl] Add NO_AUTH enum

There's going to be a use case to clear auth configuration even after
we remove the old auth stack.

Renumbering is safe because this enum is not stored outside of runtime.

Bug: b/351071334
Change-Id: Ib6522ffa6f1c8d817a37cce48d1abf2fcd7be339
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5727083
Commit-Queue: Allen Li <ayatane@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

git_cl.py

@@ -3679,9 +3679,10 @@ def ClearGitRepoAuth() -> None:
 
 class GitAuthMode(enum.Enum):
     """Modes to pass to GitAuthConfigChanger"""
-    NEW_AUTH = 1
-    NEW_AUTH_SSO = 2
-    OLD_AUTH = 3
+    NO_AUTH = 1
+    OLD_AUTH = 1  # TODO(ayatane): remove later
+    NEW_AUTH = 2
+    NEW_AUTH_SSO = 3
 
 
 class GitAuthConfigChanger(object):