Switch to squash merge for cleaner commit history

[chensun]

When I look at the change history under a folder, for example https://github.com/GoogleCloudPlatform/oss-test-infra/commits/master/prow/prowjobs/kubeflow/pipelines, it's not very easy to find a past change because the individual commits from a PR are not squashed. Can we switch to "squash merge" in the repo to have a cleaner change history?

[chaodaiG]

Yes this is supported, and I can see that kubeflow is already configured to do so

oss-test-infra/prow/oss/config.yaml

Line 31 in d99cc26

kubeflow: squash