Add rebasing to QA checklist

@jamedjo @rymai wdyt?

The reason I am adding it here is because of https://gitlab.com/gitlab-org/gitlab-ee/issues/2979, which is not super-easy to test automatically, but this is an important feature that we can verify quite easily manually.

assigned to @rymai

Related to https://gitlab.com/gitlab-org/gitlab-qa/issues/11, I've also added that to the scenario to implement in GitLab QA.

approved this merge request

merged