Add support for uploading a bunch of git commits (from multiple repos) to gerrit in one go

[twmr]

I would like to be able to

• after a refactoring in a project consisting of multiple git repos, stage all changes and commit them using a single commit message (I heard that this is already possible using pycharm). I think this should not be part of gerrit.el but of a package like e.g. magit.
• upload all the changes to gerrit in one go and include them in a topic. Does it make sense to use the infrastructure from project.el for this?