Using Gitlab for review with merge requests

Projects are now using merge requests, and that workflow has some gaps compared to their old review process. This issue is for collecting experiences and feelings on the new MR-based review in Gitlab, perhaps later to be refined into actionable items.

This issue started from the woes of a Weston maintainer, but anyone is welcome to add comments, especially if their old workflow was similar to Weston's.

For the record, Weston used to use a mailing list based review workflow. Patches got sent to the mailing list, reviewed on the mailing list by replying with interleaved comments, patches were re-sent to the mailing list, and once accepted enough, a maintainer applied them to git and pushed to the upstream repository usually rebased, without merge commits. Some had to apply the patches locally to git before being able to review in depth. Some had manual scripts to run their test suite for each commit.