Add a shared workflow to check for merge commits in a branch

Fix eclipse-platform/eclipse.platform.releng#214

.github/workflows/README.MD

@@ -0,0 +1,15 @@
+# Platform Shared Workflows
+
+This repository contains some workflows that can be shared across all platform repositories, se below for some details of the workflows.
+
+## verifyFreezePeriod.yml
+
+Check if there is currently a freeze period in place.
+
+## updateRelease.yml
+
+Workflow that can be used to trigger an automatic update to the next release when a new milestone is created
+
+## checkMergeCommits.yml
+
+Check if a PR contains unwanted merge commits.

.github/workflows/checkMergeCommits.yml

@@ -0,0 +1,22 @@
+name: Check for merge commits
+
+on:
+  workflow_call:
+
+jobs:
+  message-check:
+    name: Block Merge Commits
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: read
+    steps:
+      - name: Block Merge Commits
+        uses: Morishiri/block-merge-commits-action@a4554c78def8d874966a8d1e20e2971121443755
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Update Message
+        if: ${{ failure() }}
+        uses: actions/github-script@v3
+        with:
+          script: |
+            core.setFailed('Please always use reabase to update your branch')