Add lefthook check for changelog on first commit on branch

What does this MR do and why?

Issues a warning in Lefthook if the first commit of the branch doesn't have a Changelog git trailer 🎉

How to set up and validate locally

git checkout ddieulivol-lefthook_check_changelog
git checkout -b my-test-branch
git reset HEAD~1 # Remove the commit from the MR
git add .
git commit -m 'A commit without a changelog'
bundle exec lefthook run pre-push # should fail

git commit --amend

"""
A commit with a changelog

Changelog: added
"""

bundle exec lefthook run pre-push # should succeed

MR acceptance checklist

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