Forbid fixup/squash commits

[yrodiere]

Commits whose message starts with fixup! or squash! must be squashed before the PR is merged.

Not strictly necessary since we check that commit messages start with the key of a JIRA ticket, but that could be a nice check regardless.

See https://git-scm.com/docs/git-commit#Documentation/git-commit.txt---fixupltcommitgt