Nice! A couple of thoughts re. your "It's not perfect" which wouldn't add real extra complexity:
Using --from with git-format-patch will add the From: header to the commit message body in the standard way when emailing a patch written by someone else, which would allow you to preserve this easily without risking excitement from injection attacks into email headers.
Maybe the right behaviour for all-zero $oldsha is to quietly turn format-patch $oldsha..$newsha into format-patch --root $newsha?
Nice! A couple of thoughts re. your "It's not perfect" which wouldn't add real extra complexity:
Using --from with git-format-patch will add the From: header to the commit message body in the standard way when emailing a patch written by someone else, which would allow you to preserve this easily without risking excitement from injection attacks into email headers.
Maybe the right behaviour for all-zero $oldsha is to quietly turn format-patch $oldsha..$newsha into format-patch --root $newsha?