Use merge commit for PR instead of HEAD commit of PR (#14)

* Try to not pass explicit branch HEAD so we hopefully get the merge commit instead.

* Remove existing checkout, so we really check out HEAD.

* Another try: use the HEAD ref.

* Yet another try.

* Remove not needed step.

* Document dilemma.

Author: felixfontein

.github/workflows/_shared-docs-build-pr.yml

@@ -193,7 +193,13 @@ jobs:
       - name: Checkout HEAD
         uses: actions/checkout@v2
         with:
-          ref: ${{ github.event.pull_request.head.sha }}
+          # It would be better to use
+          #
+          #     ref: ${{ github.event.pull_request.head.sha }}
+          #
+          # since that also works when re-running this action for older commits of
+          # a PR, **but** then we get https://github.com/ansible-community/github-docs-build/issues/3 back...
+          ref: refs/pull/${{ github.event.number }}/merge
           path: ${{ steps.vars.outputs.checkout-path }}
 
       - name: Initialize the build environment (HEAD)