automatically check CONTRIBUTORS file for completeness

It is fairly easy to overlook adding new contributors to the CONTRIBUTORS file that should exist in each repo. We should augment the CI and/or our pre-commit checks to avoid that, here is a simple oneliner that would do the job (assuming a list of known SWH staffers is present in a given file, ~/swh-team.txt in the example):

git log --pretty=%an | sort -u | while read author ; do if ! grep -q "$author" ~/swh-team.txt && ! grep -q "$author" CONTRIBUTORS ; then echo "unknown author: $author" ; exit 2 ; fi ; done

---

Migrated from T1855 (view on Phabricator)