check: branch checks incorrectly run on all commits

git-checks distinguishes commit checks from branch checks. The latter are supposed to run only on the head of a topic. However, check_mr is looping over the entire topic itself and running the checks on each commit individually. This causes branch checks to be run on every commit. Instead the git-checks run method expects to be given the refs in the topic from newest to oldest, such that the .first() in the slice is the head of the topic.