feat: add Personal access token (PAT)

https://docs.github.com/en/github/authenticating-to-github/creating-a-personal-access-token

action.yml

@@ -15,6 +15,15 @@ inputs:
   branch:
     description: 'Branch to merge to. For example, master'
     default: 'master'
+  token:
+    description: >
+      Personal access token (PAT) used to fetch the repository. The PAT is configured
+      with the local git config, which enables your scripts to run authenticated git
+      commands. The post-job step removes the PAT.
+      We recommend using a service account with the least permissions necessary.
+      Also when generating a new PAT, select the least scopes necessary.
+      [Learn more about creating and using encrypted secrets](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/creating-and-using-encrypted-secrets)
+    default: ${{ github.token }}
 
 runs:
   using: "composite"
@@ -23,7 +32,8 @@ runs:
         git remote add -f upstream "https://github.com/${{ inputs.upstream }}.git"
         git remote -v
         git branch --all
+        git config --list
         git checkout ${{ inputs.branch }}
         git merge --ff-only upstream/${{ inputs.upstream-branch }}
         git push 
-      shell: bash
+      shell: bash