[systemd-devel] github merge rules update

[Zbigniew Jędrzejewski-Szmek]

To systemd repo committers:

an investigation in https://github.com/systemd/systemd/issues/8665
done by filbranden showed that github's "rebase & merge" button works
better than the "squash & merge" version. The authorship and original
timestamps on the commit are not mangled. Hence, let's mostly use
"rebase & merge" for single commits instead of "squash & merge".

New rules:
— single commit → rebase
— a bunch of fixup commits that should be one → squash
— more than one commit → merge

Zbyszek