tools: add util scripts to land and rebase PRs

I've been using those scripts for a while now, and I figured I may as well send those in a PR in case they can be helpful to someone else – although I don't expect those to be useful to most folks – and I don't lose them if I delete them by mistake.

• in case you want to land a PR without the CQ (not recommended), the script merge.sh in this PR provides an alternative to "purple-merge" a PR without force pushing its branch.
• the rebase.sh one is mostly useful when working with a very old PRs, or very large ones, when you want to rebase it without having to check it out locally. It's exactly equivalent to the "Update branch" button on the GitHub UI, that we had to disable because by default it creates a merge commit, that we never want as it breaks our tooling.