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In the rare occasion that I need to make downstream-only changes with patches, I usually just explode the upstream tarball, run "git init", then "git add .", "git commit -m import", apply my changes, and then do "git diff --patch > ../00-my-changes.patch" (if it's just one commit), or "git format-patch -o ../" if there are multiple commits, and then delete the exploded sources again.
In any case, I think this functionality could be included in rpkg/fedpkg...?
If there are no objections, I will open a ticket for this.
Maybe also something from https://pagure.io/glibc-maintainer-scripts/blob/master/f/glibc-sync-upstream... could be included too?