Check for merge commit after checking Copybara tag (#5037)
Check for merge commit after checking Copybara tag (#5037)

We don't want to try to create merge commits from things that don't have the Copybara
tag or are already merge commits, but we do still want to fix submodules. Probably if it's
a merge commit it shouldn't have messed up submodules anyway, but we want to exit
successfully if it doesn't have the Copybara tag. Otherwise we will fail on every google
export (like https://github.com/google/iree/runs/2071416701)

TESTED=yolo
google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,google/iree