Merge branch 'ap/merge-stop-at-prepare-commit-msg-failure'

"git merge" started calling prepare-commit-msg hook like "git commit" does some time ago, but forgot to pay attention to the exit status of the hook. t7505 may want a general clean-up but that is a different topic. * ap/merge-stop-at-prepare-commit-msg-failure: merge: Honor prepare-commit-msg return code