To my knowledge, stacked diffs were first done in the Linux kernel as stacks of patches sent over email. From there they spread to Google and Facebook. (Source: I worked on Facebook's source control team from 2012-2018 and did a lot of work to enable stacked diffs there.)
Right, I was thinking from a web-based UI. The "pull request" term is from git (AFAIK), but git itself was built to accommodate the earlier concept of mailing patches around. (Source: I've been using version control since RCS/SCCS days and contributed here and there to git in its infancy. Also an early user/contributor to Gerrit.)
Possibly from github. It got popularized there at least, encouraging forking code, and is why so many people say "pull request" when they mean "merge request".
GitHub took the "pull request" terminology from Git. A kernel developer (say) would have a bunch of changes ready in their local Git server and would request a pull from Linus, hence a pull request. There's literally a command for it:
At some point, a derivative idea becomes so different from the original one that it’s a novel idea in essence. Just like SMS is ultimately a derivative of cuneiform tablets, and yet it isn’t in any meaningful sense.
I don't think mailing stacks of patches is that different? As someone who built this stuff it was pretty obvious to me that web-based patch stack management was a relatively small evolution over mailing lists. Tools like patchwork bridged the gap initially, and we were quite familiar with them.
Ahem, pioneered by gerrit. But actually, I'm almost certain even that wasn't original art. I think gerrit just brought it to git.
https://www.gerritcodereview.com/about.html