[git_auth] Add comment about import cycle

Bug: b/351071334
Change-Id: I14aa44e81d936201c9ddf0ffc52152287ec54cad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5742789
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Allen Li <ayatane@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

git_auth.py

@@ -15,7 +15,8 @@ import newauth
 import scm
 
 if TYPE_CHECKING:
-  import git_cl
+    # Causes import cycle if imported normally
+    import git_cl
 
 
 class ConfigMode(enum.Enum):