Don't keep auto-submit on all new patchsets

Better to not automatically submit when a new patchset has changes than
to accidentally submit because somebody forgot to clear auto-submit.

Change-Id: Ide8404ec16db5b9a676d0f41ee632edd62d97b5a
Reviewed-on: https://pigweed-review.googlesource.com/c/Public-Commit-Queue-Projects/+/141972
Reviewed-by: Carlos Chinchilla <cachinchilla@google.com>