Re: [PATCH 7/7] shortlog: implement `--group=trailer` in terms of `--group=<format>`

> I spent a fair bit of time optimizing the regular author/committer paths
> (and I think if we restore the skip-the-dedup logic I mentioned earlier
> for those, they should be equivalent, as they already use
> format_commit_message()). But the internals of the trailer code are
> already so slow I doubt it makes much of a difference either way here.
> 
> The one thing that could hurt is that multiple trailers now require
> multiple format calls, which may load the commit object for each one. I
> think _probably_ not because we'd hopefully have it cached in the commit
> slab via the save_commit_buffer mechanism. Though I guess if we used the
> commit graph in the first place, we would need to load it. We'd also
> likely reencode, but that's a noop on utf8 commits, so it's probably not
> worth worrying about.