CI: workaround shallow PR history

Author: bsipocz

tox.ini

@@ -53,7 +53,7 @@ commands =
     !buildhtml: bash -c 'if [[ $CI == true ]]; then cat ignore_gha_testing >> ignore_testing; fi'
 
     # We only want to run CI in PRs for the notebooks we touched
-    !buildhtml: bash -c 'if [[ $GITHUB_EVENT_NAME == pull_request ]]; then git diff origin/main --name-only tutorials | grep .md; else find tutorials -name "*.md"; fi | grep -vf ignore_testing | xargs jupytext --to notebook '
+    !buildhtml: bash -c 'if [[ $GITHUB_EVENT_NAME == pull_request ]]; then git fetch origin main --depth=1; git diff origin/main --name-only tutorials | grep .md; else find tutorials -name "*.md"; fi | grep -vf ignore_testing | xargs jupytext --to notebook '
 
     !buildhtml: pytest --nbval-lax -vv --durations=10 tutorials
     buildhtml: sphinx-build -b html . _build/html -D nb_execution_mode=auto -nWT --keep-going