This prohibits commits whose commit message starts with "squash!",
"fixup!", or "amend!" unless the PR has the 'pr-permit-autosquash'
label. Such commits are created via the `--fixup` or `--squash`
options to `git commit`, and cause `git rebase --autosquash` to
automatically adjust the todo list appropriately before performing the
rebase. Their existence implies that the PR should be rebased with
`--autosquash` before merging.
When combined with GitHub's "Require branches to be up to date before
merging" setting, this forces semi-linear merging. This check can be
disabled by adding the "pr-permit-nonlinear" label to the PR.
