Someone correct me if I'm wrong, but if my memory is correct, I was under the impression that a pull request was a term coined in the early Github days specific to their PR feature.
Yes, diff is a generic name, after a Unix tool with the same name. However, it hadn’t been the most popular name for the self contained unit of changes, that you would send to others to review or apply — the “patch” was. In Linux git workflow, you would use the “git format-patch” command to generate patch to send to others for review and to apply it to their branch.
However, even if you're using an email based workflow you're still making a request to somebody to pull your changes into their tree, and I'm reasonably sure 'pull request' as a term for that came from the linux workflow. GH more provided features to formalise a common concept (which, even where I've disagreed with them on the details of how they formalised it, has definitely been a net win overall).
Someone correct me if I'm wrong, but if my memory is correct, I was under the impression that a pull request was a term coined in the early Github days specific to their PR feature.