try ref_name, as head_ref doesn't work cross repository

.github/workflows/ci.yml

@@ -14,11 +14,11 @@ jobs:
       - name: Fetch all commits for PR branch plus head commit of base branch
         run: |
           # fetch all commits of the PR branch
-          git fetch --shallow-exclude "${{ github.base_ref }}" origin "${{ github.head_ref }}"
+          git fetch --shallow-exclude "${{ github.base_ref }}" origin "${{ github.ref_name }}"
           # fix for "fatal: error in object: unshallow"
           git repack -d
           # fetch head commit of base branch
-          git fetch --deepen 1 origin "${{ github.head_ref }}"
+          git fetch --deepen 1 origin "${{ github.ref_name }}"
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: 2.6