In this workflow, I don't think it's meant to have the same SHA1 digest. It's a workflow that's very much designed for a handful of core contributors (who have direct repository access) and gatekept one-off patches.

Some other accepted git workflows, like rebasing onto master, or even adding a "committed-by" or "signed-off-by", don't preserve SHA1 hashes either, and it seems you don't really need that property outside of closely collaborating cliques.

I get your point and maybe my tone was snarky (not a native speaker). But why would you want an exact reproduction on the other side? The diff format is human-readable for a reason, so slight errors can be fixed quite easily (if they do happen). Extracting patches from a well-configured MUA can be done quickly too.