Git-am applies commit message diffs

There are some wild skeletons in that git closet:

https://lore.kernel.org/git/20260206092423.GB2761602@coredum...

> we did have a problem with fidelity of commit message in git-rebase, since it was based on a format-patch/am pipeline. And we solved it by teaching "git am" a magic "--rebasing" flag