backport: #1099

	name: Check Merge Fast-Forward Only

	on:
	push:
	pull_request:

	jobs:
	check_merge:
	runs-on: ubuntu-latest
	steps:
	- name: Checkout repository
	uses: actions/checkout@v2
	with:
	fetch-depth: 0

	- name: Set up Git
	run: \|
	git config user.name "GitHub Action"
	git config user.email "[email protected]"

	- name: Check merge --ff-only
	run: \|
	git fetch origin master:master
	git checkout master
	if [ "${{ github.event_name }}" == "pull_request" ]; then
	git merge --ff-only ${{ github.event.pull_request.head.sha }}
	else
	git merge --ff-only ${{ github.sha }}
	fi

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