[Feature] Add flag to disable auto merge

- Once the auto merge is disabled, will command on the PR to remind
  users this PR branch is not up to date.

Signed-off-by: Kent Huang <kent@infuseai.io>

Author: kentwelcome

action.yml

@@ -6,6 +6,21 @@ inputs:
     required: true
     description: 'The base branch to compare and merge.'
 
+  autoMerge:
+    required: false
+    description: 'Automatically merge the base branch into the pull request branch if necessary.'
+    default: 'true'
+
+  descriptionMerged:
+    required: false
+    description: 'Description of the merge commit.'
+    default: 'Merge ${{ github.event.pull_request.base.ref }} into ${{ github.event.pull_request.head.ref }}'
+
+  descriptionReminder:
+    required: false
+    description: 'Description of the PR comment if 'autoMerge' is false.'
+    default: 'Please merge `${{ github.event.pull_request.base.ref }}` into `${{ github.event.pull_request.head.ref }}`.'
+
 runs:
   using: 'composite'
   steps:
@@ -28,29 +43,43 @@ runs:
         BASE=$(git merge-base origin/${{ github.event.pull_request.base.ref }} HEAD)
         if [ $BASE = $(git rev-parse origin/${{ github.event.pull_request.base.ref }}) ]; then
           echo "Branches are up to date, skipping merge."
-          echo "::set-output name=mergeRequired::false"
+          echo "mergeRequired=false" >> $GITHUB_OUTPUT
         else
           echo "Branches are not up to date, merge is required."
-          echo "::set-output name=mergeRequired::true"
+          echo "mergeRequired=true" >> $GITHUB_OUTPUT
         fi
 
     - name: Merge base branch into PR branch
       shell: bash
-      if: steps.checkBranches.outputs.mergeRequired == 'true'
+      if: steps.checkBranches.outputs.mergeRequired == 'true' && inputs.autoMerge == 'true'
       run: |
         git merge --no-edit --no-commit origin/${{ github.event.pull_request.base.ref }}
         if [ $? -ne 0 ]; then
           echo "Merge conflict detected. Please resolve conflicts before merging."
           exit 1
         fi
-        git commit -m "Merge ${{ github.event.pull_request.base.ref }} into ${{ github.event.pull_request.head.ref }}"
+        git commit -m "${{ inputs.descriptionMerged }}"
 
     - name: Push changes
       shell: bash
-      if: steps.checkBranches.outputs.mergeRequired == 'true'
+      if: steps.checkBranches.outputs.mergeRequired == 'true' && inputs.autoMerge == 'true'
       run: |
         git push origin ${{ github.event.pull_request.head.ref }}
 
+    - name: Command on PR to remind the PR branch is not up to date
+      if: steps.checkBranches.outputs.mergeRequired == 'true' && inputs.autoMerge == 'false'
+      uses: thollander/actions-comment-pull-request@v3.0.1
+      with:
+        message: |
+          ## This branch is out-of-date with the base branch
+          ${{ inputs.descriptionReminder }}
+        comment-tag: pr-up-to-date
+
+    - name: Block the workflow if the PR branch is not up to date
+      shell: bash
+      if: steps.checkBranches.outputs.mergeRequired == 'true' && inputs.autoMerge == 'false'
+      run: exit 1
+
 branding:
   icon: 'git-pull-request'
   color: 'blue'