Add action that checks for autosquash commits in pull requests

We want to block merging for these, to ensure that you autosquash
before merging.

Ideally we'd like automatic (or at least via a bot) autosquashing, but
that is not currently possible.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f