Add GitHub Action to automatically create cherrypick PRs (#10361)

* Add GitHub Action to automatically create cherrypick PRs

Uses peter-evans/create-pull-request and actions/checkout,
both of which are already dependencies.

When merging a PR with a cherrypick label, which only contains
a single commit, attempt to cherrypick the single commit into
a new branch, then create a PR.

Fails if there are multiple commits (to catch squash and merge).
Fails if the cherrypick does not apply cleanly.

Does not attempt to merge the new PR.
Does not attempt to update stable.

* Handle squash and merge

* Apply suggestions from code review

Co-authored-by: Thaddeus Crews <repiteo@outlook.com>

---------

Co-authored-by: Thaddeus Crews <repiteo@outlook.com>

.github/workflows/cherrypick.yml

@@ -0,0 +1,71 @@
+name: Create Cherrypick PR
+
+on:
+  pull_request:
+      types:
+        - closed
+      branches:
+        # TODO: Extract this to an env variable?
+        - 'master'
+
+env:
+  # TODO: Add a way to handle multiple potential cherrypick targets.
+  TARGET_BRANCH: '4.3'
+  USERNAME: 'Godot Organization'
+  EMAIL: 'noreply@godotengine.org'
+
+jobs:
+  Create-cherrypick-PR:
+    # The cherrypick label is hardcoded because `contains()` doesn't seem to be able to use an environment variable as a second argument.
+    if: ${{ github.event.pull_request.merged == true && contains( github.event.pull_request.labels.*.name, 'cherrypick:4.3' ) }}
+    runs-on: ubuntu-latest
+    env:
+      # "Ternary" hack featured in the official docs.
+      # When using "Squash and merge", the commit hash is the last merge commit of the pull request merge branch.
+      # When using "Merge", the commit hash is the last commit to the head branch of the pull request.
+      # This is mildly error-prone, since in theory we could merge multiple commits without squashing.
+      # We are relying on human review of the generated PRs to catch that.
+      COMMIT_HASH: ${{ github.event.pull_request.commits > 1 && github.sha || github.event.pull_request.head.sha }}
+      PR_NUMBER: ${{ github.event.number }}
+
+    steps:
+  
+    - name: Checkout
+      uses: actions/checkout@v4
+      with:
+        ref: ${{ env.TARGET_BRANCH }}
+
+    - name: Cherrypick Commit
+      id: cherrypick_commit
+      continue-on-error: true
+      # TODO: Maybe only fetch some branches?
+      run: |
+        git config user.name "${{ env.USERNAME }}"
+        git config user.email "${{ env.EMAIL }}"
+        git fetch
+        git cherry-pick -m 1 ${{ env.COMMIT_HASH }}
+
+    - name: Create Pull Request
+      if: steps.cherrypick_commit.outcome == 'success'
+      uses: peter-evans/create-pull-request@v7
+      with:
+        commit-message: 'Cherrypick to ${{ env.TARGET_BRANCH }}'
+        branch: 'cherrypick-${{ env.PR_NUMBER }}-${{ env.TARGET_BRANCH }}'
+        delete-branch: true
+
+        # Configure the commit author.
+        author: '${{ env.USERNAME }} <${{ env.EMAIL }}>'
+        committer: '${{ env.USERNAME }} <${{ env.EMAIL }}>'
+
+        # Configure the pull request.
+        title: 'Cherrypick ${{ env.PR_NUMBER }} to ${{ env.TARGET_BRANCH }}'
+        body: 'Cherrypick #${{ env.PR_NUMBER }} to ${{ env.TARGET_BRANCH }}.'
+        # TODO: Only add the bug or enhancement label, depending on which the original PR uses.
+        labels: 'bug,enhancement'
+
+    - name: Handle failure
+      if: steps.cherrypick_commit.outcome == 'failure'
+      run: |
+        echo "Can't automatically cherrypick. Potential causes:"
+        echo "- PR has multiple commits. Did you squash and merge?"
+        echo "- Cherrypick did not apply cleanly and can't be auto-merged."