Prototype of pull request workflow for Gerrit

[Dave Borowitz]

At the hackathon, I gave a quick demo of a prototype I made over Christmas break. Here is the code:

https://gerrit-review.googlesource.com/c/gerrit/+/224397

The presentation was also recorded, and hopefully we're able to share the videos soon.

For this prototype, I started from the question: how can we allow users to push arbitrary commits with `git push origin my-feature-branch` and turn those commits into a single code review? The result is something I called "branch changes" and which shares many similarities with the "pull request" workflow used by systems like GitHub. Surprisingly, this required relatively minimal changes to Gerrit's core Change data model, as detailed in the commit message. There are of course different ways of providing review of multiple commits as a single unit (e.g. creating and reviewing a merge commit as a standard Gerrit change), but what is unique about this approach is that users never have to use any commands that they can't find in a standard non-Gerrit Git tutorial.

I want to stress that this is a prototype, there are a lot of missing features and big unknowns that haven't yet been thought through, and it is currently not on Google's roadmap to implement this. That said, I also have the beginnings of a design doc that I will also share later this week.

[Dave Borowitz]

As promised, here is my draft design doc:

https://docs.google.com/document/d/1xsUv5eTvD0MjDvzBzrE-FVQdKTjI9YyT0HJIsLaszHg/edit#

[Paladox none]

This is awesome supporting code review alongside pull requests!

Thank you @Dave for working on this! The design doc is also excellent!