build: fix landed message for multiple commits in commit-queue

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Anyone knows of a better way than git merge-base?

Refs: https://github.com/nodejs/node/issues/34770#issuecomment-693201987

/cc @mmarchini @targos