batches: updateChangeset can panic on Bitbucket Cloud when not on a fork

Created by: LawnGnome

Rather than rehashing this, see #37649 for an analysis of the cause of this.

tl;dr: the implementation of ChangesetSource.UpdateChangeset for Bitbucket Cloud assumed that sources.Changeset.RemoteRepo would always be set, but executor.updateChangeset didn't always do so.