chore: Adding PR commit check github workflow

This will fail a PR check if multiple commits are added, and will give the user instructions on how to fix the issue.

PiperOrigin-RevId: 776638574

Author: hangfei

.github/workflows/pr-commit-check.yml

@@ -0,0 +1,62 @@
+# .github/workflows/pr-commit-check.yml
+# This GitHub Action workflow checks if a pull request has more than one commit.
+# If it does, it fails the check and instructs the user to squash their commits.
+
+name: 'PR Commit Check'
+
+# This workflow runs on pull request events.
+# It's configured to run on any pull request that is opened or synchronized (new commits pushed).
+on:
+  pull_request:
+    types: [opened, synchronize]
+
+# Defines the jobs that will run as part of the workflow.
+jobs:
+  check-commit-count:
+    # The type of runner that the job will run on. 'ubuntu-latest' is a good default.
+    runs-on: ubuntu-latest
+
+    # The steps that will be executed as part of the job.
+    steps:
+      # Step 1: Check out the code
+      # This action checks out your repository under $GITHUB_WORKSPACE, so your workflow can access it.
+      - name: Checkout Code
+        uses: actions/checkout@v4
+        with:
+          # We need to fetch all commits to accurately count them.
+          # '0' means fetch all history for all branches and tags.
+          fetch-depth: 0
+
+      # Step 2: Count the commits in the pull request
+      # This step runs a script to get the number of commits in the PR.
+      - name: Count Commits
+        id: count_commits
+        # We use `git rev-list --count` to count the commits.
+        # ${{ github.event.pull_request.base.sha }} is the commit SHA of the base branch.
+        # ${{ github.event.pull_request.head.sha }} is the commit SHA of the head branch (the PR branch).
+        # The '..' syntax gives us the list of commits in the head branch that are not in the base branch.
+        # The output of the command (the count) is stored in a step output variable named 'count'.
+        run: |
+          count=$(git rev-list --count ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }})
+          echo "commit_count=$count" >> $GITHUB_OUTPUT
+
+      # Step 3: Check if the commit count is greater than 1
+      # This step uses the output from the previous step to decide whether to pass or fail.
+      - name: Check Commit Count
+        # This step only runs if the 'commit_count' output from the 'count_commits' step is greater than 1.
+        if: steps.count_commits.outputs.commit_count > 1
+        # If the condition is met, the workflow will exit with a failure status.
+        run: |
+          echo "This pull request has ${{ steps.count_commits.outputs.commit_count }} commits."
+          echo "Please squash them into a single commit before merging."
+          echo "You can use git rebase -i HEAD~N"
+          echo "...where N is the number of commits you want to squash together. The PR check conveniently tells you this number! For example, if the check says you have 3 commits, you would run: git rebase -i HEAD~3."
+          echo "Because you have rewritten the commit history, you must use the --force flag to update the pull request: git push --force"
+          exit 1
+
+      # Step 4: Success message
+      # This step runs if the commit count is not greater than 1 (i.e., it's 1).
+      - name: Success
+        if: steps.count_commits.outputs.commit_count <= 1
+        run: |
+          echo "This pull request has a single commit. Great job!"