github actions: Allow fork checkout for commit validation

Since we have external contributors we need process each of the commits
for faster reviews, validation that header information is correct and to
check the state of our tickets so that is actually tracked correctly
internally, on behalf of some external contributors.

Author: PlaidCat

.github/workflows/validate-kernel-commits.yml

@@ -14,15 +14,23 @@ jobs:
     timeout-minutes: 120
 
     steps:
-      - name: Checkout PR branch
+      - name: Checkout base branch
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.base.ref }}
 
-      - name: Checkout base branch
+      - name: Add and checkout PR branch
         run: |
-          git fetch origin ${{ github.base_ref }}:${{ github.base_ref }}
+          set -e  # Exit on error
+          if [ "${{ github.event.pull_request.head.repo.full_name }}" = "${{ github.event.pull_request.base.repo.full_name }}" ]; then
+              git fetch origin ${{ github.event.pull_request.head.ref }}
+              git checkout FETCH_HEAD
+          else
+              git remote add fork-pr https://github.com/${{ github.event.pull_request.head.repo.full_name }}
+              git fetch fork-pr ${{ github.event.pull_request.head.ref }}
+              git checkout FETCH_HEAD
+          fi
 
       - name: Checkout kernel-src-tree-tools
         uses: actions/checkout@v4