Reset changeset detection when PR base branch is changed

[pcattori]

In my repo, if I create a mybranch branch off of my dev branch, then make a PR with mybranch but accidentally keep the base as main, then the changeset bot correctly detects changesets that are in dev, but not yet in main.

However, when I then correct my mistake and change the base of the PR to dev, the changeset bot does not try to revalidate, so keeps saying there are changesets in my PR when in fact there aren't any.

If the changeset bot could react to a PR changing its base and redetect changesets, this issue would be fixed.