Strive for non overlapping branch history

See this IBIS for the conversation.

Tangled branch history are a hassle for maintainers. Full linearity suppresses detail that can be informative.

We strive for non-overlapping branch history on our master branch on github. Occasional short exceptions are not a big deal.

Mergifiy's automerge:squash and automerge:rebase produce clean history. automerge:merge does as well as long as the branch's last rebase to master isn't very old.