feat(ci): Add linear-history and no-fixups checks

Author: kriskowal

Diff for: .github/workflows/ci.yml

@@ -398,3 +398,76 @@ jobs:
         run: |
           PATH=$PATH:$GITHUB_WORKSPACE/bin
           yarn test:xs
+
+  linear-history:
+    runs-on: ubuntu-latest
+    if: >-
+      github.event_name == 'pull_request' &&
+      github.event.pull_request.draft == false &&
+      github.event.pull_request.base.ref == 'master' &&
+      !contains(github.event.pull_request.labels.*.name, 'bypass:linear-history')
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - shell: bash
+        env:
+          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          HEAD_LABEL: ${{ github.event.pull_request.head.label }}
+          BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          BASE_LABEL: ${{ github.event.pull_request.base.label }}
+        run: |
+          merge_commits=$(git rev-list --merges "$BASE_SHA".."$HEAD_SHA")
+
+          if [ -n "$merge_commits" ]; then
+            echo "Error: merge commits found in $BASE_LABEL..$HEAD_LABEL"
+
+            for merge_commit in $merge_commits; do
+              echo "$merge_commit"
+            done
+
+            exit 1
+          fi
+
+          fixup_commits=
+          for commit in $(git rev-list $BASE_SHA..$HEAD_SHA); do
+            case $(git show --pretty=format:%s -s $commit) in fixup\!*|squash\!*|amend\!*)
+              fixup_commits="$fixup_commits\n$commit"
+              ;;
+            esac
+          done
+
+          if [ -n "$fixup_commits" ]; then
+            echo "Error: fixup/squash/amend commits found in $BASE_LABEL..$HEAD_LABEL"
+            echo -e "$fixup_commits"
+            exit 1
+          fi
+
+  no-fixup-commits:
+    runs-on: ubuntu-latest
+    if: >-
+      github.event_name == 'pull_request' &&
+      github.event.pull_request.draft == false &&
+      github.event.pull_request.base.ref == 'master' &&
+      !contains(github.event.pull_request.labels.*.name, 'bypass:linear-history')
+
+    env:
+      HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+      BASE_SHA: ${{ github.event.pull_request.base.sha }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: Check for fixup commits
+        id: fixup-commits
+        run: |
+          if ! git merge-base --is-ancestor "$BASE_SHA" "$HEAD_SHA"; then
+             echo "PR is not up to date with target branch, skipping fixup commit check"
+          elif [[ $(git rev-list "$BASE_SHA".."$HEAD_SHA" --grep="^\(fixup\|amend\|squash\)! " | wc -l) -eq 0 ]]; then
+            echo "No fixup/amend/squash commits found in commit history"
+          else
+            echo "fixup/amend/squash commits found in commit history"
+            exit 1
+          fi