DM-46049: Add a Slack webhook to notify on failure #56

	name: "Check that the 'lsst-dev' branch was not merged into the development branch"
	on: pull_request

	jobs:
	rebase-checker:
	runs-on: ubuntu-latest

	steps:
	- name: Check that 'lsst-dev' is not merged into the development branch
	uses: gsactions/commit-message-checker@v2
	with:
	excludeDescription: "true" # optional: this excludes the description body of a pull request
	excludeTitle: "true" # optional: this excludes the title of a pull request
	checkAllCommitMessages: "true" # optional: this checks all commits associated with a pull request
	accessToken: ${{ secrets.GITHUB_TOKEN }} # github access token is only required if checkAllCommitMessages is true
	# Check for patterns that emerge from
	# 1) git pull origin lsst-dev, as well as
	# 2) hitting "Update branch" button on GitHub.
	pattern: ^(?!Merge branch 'lsst-dev')
	error: \|
	"This step failed because you merged the 'lsst-dev' branch into the development branch,
	likely by clicking the 'Update branch' button on the GitHub pull request page.

	In order to bring this pull request to a mergeable state,
	update the 'lsst-dev' branch locally and rebase against it.
	See https://developer.lsst.io/work/flow.html#pushing-code for detailed instructions.

	To avoid this error in the future, rebase against the latest 'lsst-dev' branch
	by following the instructions above, or
	use the little down arrow on the right side of 'Update branch' and click
	'Update with rebase' option."

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