Better handling of MR's authors
We are currently using the author of the merge commits while we should actually go through the commits inside a merged branch and to get their authors.
It's probably easier to figure that out that using the gitlab MR's author + commits (as per https://python-gitlab.readthedocs.io/en/v1.11.0/gl_objects/mrs.html) though instead of trying to figure it out via git, given that the latter isn't trivial.