stack-merge feedback: If CI is bypassed by admin at GitHub, git-spice gets stuck

If, midway through a `git-spice stack merge`, a GitHub admin skips CI checks and merges the bottom-most PR into main, git-spice keeps waiting indefinitely.

Perhaps git-spice should be idempotent. If the merge happens outside of the CLI, it should be detected and accepted as a success, and move on to the next PR in the stack.

We use this override to avoid running redundant CI checks during squash+merge of multiple PRs