scripts: add backporting script

This adds a simple script for backporting pull requests to older
branches. It accepts as parameters a list of pull request numbers which
whose commits are to be cherry-picked. The identification of PRs
currently happens by using the commit message of the merge of the PR,
which should conform to the message "Merge pull request #<PR>".

While the heuristic works in practice, we could instead also use the
direct references from GitHub via "pull/#<PR>/head". This requires the
user to have all these references fetched, though, so we can just use
the current heuristic until we experience any issues with that.

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