auto-merge-squash drops all commit messages

[MartinNowak]

Github's squash and merge button suggests all of the commit titles, but strips merge requests. Not sure about [squash] and !fixup commit messages, but would be sensible to strip those as well.

[MartinNowak]

Has been disabled for now by removing the auto-merge-squash labels. Should stay removed until dlang-bot learns to synthesized the correct commit message or can do rebase --autosquash itself.

[CyberShadow]

Should we just include a list of commits and their commit messages in the merge commit message, then?

[Geod24]

Usually we use squash when a contributor doesn't have a proper commit separation - e.g. create 30 commits as the result of review. So IMO we should pick the PR title / body instead.