diff with upstream branch if no previous commit

.github/workflows/tex-pdf.yml

@@ -33,12 +33,12 @@ jobs:
       - name: Get changed files
         run: |
             # check to see if there's a previous commit on the branch
-            # if not, then diff with the common ancestor of HEAD and origin/main
+            # if not, then diff with the upstream branch
             # if yes, then diff with the HEAD @ prevous commit
             if [[ -z "${{ github.event.before_ref }}" ]] ; then
-              export MERGE_BASE_SHA=$(git merge-base origin/main ${{ github.sha }})
-              export DIFF=$( git diff --name-only $MERGE_BASE_SHA ${{ github.sha }} )
-              echo "Diff between $MERGE_BASE_SHA and ${{ github.sha }} "
+              export UPSTREAM_BRANCH=$(git rev-parse --abbrev-ref --symbolic-full-name @{u})
+              export DIFF=$( git diff --name-only $UPSTREAM_BRANCH ${{ github.sha }} )
+              echo "Diff between $UPSTREAM_BRANCH and ${{ github.sha }} "
             else
               export DIFF=$( git diff --name-only ${{ github.event.before }} ${{ github.sha }} )
               echo "Diff between ${{ github.event.before }} and ${{ github.sha }} "