Feature Request: Make DependaBot re-add existing labels from the superseded Pull Request

Hi there,

Thanks for DependaBot, we've been using it extensively and loving it so far!

I noted that when a PR is superseded by another one, the assignees are copied from the previous one. This is a pretty useful feature, so we don't have to keep manually re-adding the same assignees when a new version is released.

I noted this isn't true for labels, though. I think it makes totally sense to also copy the labels.

(I haven't really checked about reviewers, but I think it should be copied, too).

The issue provides no file or test names; start by tracing the handling of a superseded Pull Request in this Ruby repository. Confirm the intended scope against the existing assignee-copy behavior, and define done as labels being re-added to the replacement PR, with reviewer copying included only if explicitly decided.

Written by the indexing model from the issue text.