[RFC][premerge] enabling pull-requests from a bot user

I propose allowing a special bot user creation of PRs in the main repo. Requests will be created based on diff(chain) from Phabricator and will contain a clear disclaimer that review happens on Phabricator. Commits will be created in a fork, in the main repo we will only create a PR for the lifetime of the Phabricator review.

Why?

  1. That will enable us to use standard CI (after the creation of PR) and have a setup ready for the time when we move review to PRs as well.
  2. We can get a setup when change will be submitted automatically right after CI will pass - so that the author does not need to monitor bots (if they care).
  3. It would be trivial to checkout a change under review locally to play with it.

We are likely to implement automated PR creation anyway in a fork during the refactoring of premerge tests. I would like to know your opinion about doing that in the main repo - naturally after carefully testing everything in a copy.

Phabricator does not seem to always expose the author of a diff (at least when I use arc patch): how do you plan to ensure that the commit in your fork has the right authorship?