Continuous releasing add command to rebase and force push

[jlewi]

Follow on to #450

Right now if a PR is already open, the auto update application won't submit a Tekton pipeline.

In general that is what we want. However, in certain cases we might want to rebase on master in order to pull in other changes that might fix tests.

One way to do this is just to close the PR; but then we lose the conversation thread which could be useful for debugging.

We might want to add a chat ops command "e.g. /rebase" to cause the PR to be rebased and updated.

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