Add check-rebased Github action (#2002)

It will prevent merging in a branch that's not based on its base branch HEAD, leading to streamlined history.

Note it will not prevent squash commits, nor commits directly to base branch.

.github/workflows/check-rebased.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+set -x -e -u
+
+git fetch origin "$GITHUB_BASE_REF"
+
+BASE_ID=$(git rev-parse --verify "origin/$GITHUB_BASE_REF")
+CURRENT_ID=$(git rev-parse --verify "HEAD")
+
+# Oldest commit that's not in the base branch
+FIRST_BRANCH_ID=$(git rev-list "origin/$GITHUB_BASE_REF..HEAD" | tail -n 1)
+
+# Common ancestor (usually just parent of $FIRST_BRANCH_ID)
+# Command will return error 1 if not found anything. So we || true to proceed.
+FORK_POINT_ID=$(git merge-base "$FIRST_BRANCH_ID" "origin/$GITHUB_BASE_REF") || true
+
+if [[ -z "$FORK_POINT_ID" ]]
+then
+  echo "Current branch is not forked from its base branch origin/$GITHUB_BASE_REF"
+  exit 1
+fi
+
+if [[ "$BASE_ID" != "$FORK_POINT_ID" ]]
+then
+  echo "Current branch (at $CURRENT_ID) forked at $FORK_POINT_ID is not $BASE_ID (which is the latest \"$GITHUB_BASE_REF\")"
+  exit 1
+fi

.github/workflows/check-rebased.yml

@@ -0,0 +1,17 @@
+name: check-rebased
+on:
+  pull_request:
+    types: [ synchronize, opened, reopened, edited ] # +edited (for triggering on PR base change)
+  push:
+    branches: [ main ]
+jobs:
+  hook:
+    name: Check whether current branch is based on its base branch
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repo @ current branch
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 1000 # Hopefully current branch is less than 1000 commits from main
+      - name: Run check-rebased.sh
+        run: .github/workflows/check-rebased.sh