add automerge github action

Author: vogdb

.github/workflows/automerge.yml

@@ -0,0 +1,50 @@
+# Automatically merge pull requests opened by web3-bot, as soon as (and only if) all tests pass.
+# This reduces the friction associated with updating with our workflows.
+
+on: [ pull_request ]
+name: Automerge
+
+jobs:
+  automerge-check:
+    if: github.event.pull_request.user.login == 'web3-bot'
+    runs-on: ubuntu-latest
+    outputs:
+      status: ${{ steps.should-automerge.outputs.status }}
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Check if we should automerge
+        id: should-automerge
+        run: |
+          for commit in $(git rev-list --first-parent origin/${{ github.event.pull_request.base.ref }}..${{ github.event.pull_request.head.sha }}); do
+            committer=$(git show --format=$'%ce' -s $commit)
+            echo "Committer: $committer"
+            if [[ "$committer" != "[email protected]" ]]; then
+              echo "Commit $commit wasn't committed by web3-bot, but by $committer."
+              echo "::set-output name=status::false"
+              exit
+            fi
+          done
+          echo "::set-output name=status::true"
+  automerge:
+    needs: automerge-check
+    runs-on: ubuntu-latest
+    # The check for the user is redundant here, as this job depends on the automerge-check job,
+    # but it prevents this job from spinning up, just to be skipped shortly after.
+    if: github.event.pull_request.user.login == 'web3-bot' && needs.automerge-check.outputs.status == 'true'
+    steps:
+      - name: Wait on tests
+        uses: lewagon/wait-on-check-action@bafe56a6863672c681c3cf671f5e10b20abf2eaa # v0.2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          wait-interval: 10
+          running-workflow-name: 'automerge' # the name of this job
+      - name: Merge PR
+        uses: pascalgn/automerge-action@741c311a47881be9625932b0a0de1b0937aab1ae # v0.13.1
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+          MERGE_LABELS: ""
+          MERGE_METHOD: "squash"
+          MERGE_DELETE_BRANCH: true