Rebasing instead of merging

[maximeborges]

I propose to switch to rebasing by default, so we get a nice and clean commit history which will be easier to navigate through.
Most of the PRs we get are just a few commits, most of time actually just one commit.
Having merge commits double that number for nothing, and makes commit graphs actually even worse to look at.

[eldruin]

Sounds good to me, let's do that.