如何从bzr post-commit钩子获取提交消息？

I'm trying to write a bzr post-commit hook for my private bugtracker, but I'm stuck at the function signature of post_commit(local, master, old_revno, old_revid, new_revno, mew_revid). How can I extract the commit message for the branch from this with bzrlib in Python?I'm trying to write a bzr post-commit hook for