Support for extra pre-merge checks

Created by: RalfJung

One key feature of bors/homu for GitHub is that one can set up CI such that the pre-merge (post-approval) checks performed by the bot are more extensive than the regular PR checks. This is useful in particular when there is not enough CI capacity to run the entire test suite for each push to an MR. bors/homu achieve that by using a staging branch instead of relying on PR CI to determine if a PR is good; CI config can then be set up to perform extra checks on pushes to the staging branch. (The concept of a staging branch also nicely solves https://github.com/smarkets/marge-bot/issues/111.)

It looks like marge-bot has no way to achieve the same, i.e., if I want to have extensive and expensive pre-landing checks, I have to set up the same checks to be run for each push to each MR. Is that right? In our case that would go beyond our available CI capacity, sadly.