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

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