tools: abort CQ session when landing several commits

Most PRs are meant to be squashed in one commit when landing. If the
collaborator hasn't been using `fixup!` commits, the CQ lands the PR
as several commits. This change makes the CQ abort by default
when attempting to land several commits, unless there's another
label added to the PR to force squashing or landing as several commits.

Fixes: https://github.com/nodejs/node/issues/40436
Refs: https://github.com/nodejs/node-core-utils/pull/572

PR-URL: https://github.com/nodejs/node/pull/40577


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>