Check for stacked changes dogfood var.

Bug:b/265929888
Change-Id: Ibf1203994dea6e9db72fe4a954a2f6f24e30105b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4197875
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

git_cl.py

@@ -4779,8 +4779,9 @@ def CMDupload(parser, args):
     # Load default for user, repo, squash=true, in this order.
     options.squash = settings.GetSquashGerritUploads()
 
-  if options.stacked_exp:
-    orig_args.remove('--stacked-exp')
+  if os.environ.get('DOGFOOD_STACKED_CHANGES') == '1':
+    if options.dependencies:
+      parser.error('--dependencies is not available for this workflow.')
 
     UploadAllSquashed(options, orig_args)
     return 0