Rethink git merge strategy

So currently accepted pull requests are simply merged into master. This is imho ugly for several reasons:

1. Git bisect becomes hard to use. Often I am interested in the PR that broke something, not in a specific commit.
2. I tend to work with many small commits (which is often described as a best practice for git). I then rebase my fork regularly on master. This way, I can open PRs for larger changes very early which allows reviewers to discuss it early in the process. However, when such a large pull-request gets merged, people will see many many commits from me at the top of the history
3. It looks ugly and makes the history hard to read.

Instead I would suggest to use a rebase+merge commit strategy. This would solve all of above problems and would make the git history much easier to read.

The issue names no files, tests, or entry points. Start by reviewing the repository's current pull-request merge process and the proposed rebase-plus-merge approach; done means an agreed, documented merge policy is adopted.

Written by the indexing model from the issue text.