Merge a stack that sits on another stack without dismissing approvals

[mgrabina]

We chain stacks on each other (stack B's bottom PR is based on stack A's top branch). When stack A squash-merges into a squash-only main, B's bottom PR is retargeted and its merge base changes, so a "dismiss stale approvals" ruleset dismisses its approval and the merge stops there, even though its patch is unchanged. gh stack link refuses PRs that already belong to another stack, so the stacks can't be merged as one. It would help if gh stack merge could take several chained stacks in one atomic run, or if link could append one stack onto another.