test PRs against base ref

rather than making assumptions about checkouts and origins

.github/workflows/test-docs.yml

@@ -82,9 +82,10 @@ jobs:
       # git references, and forks can be outdated, we only run these steps when
       # working against jupyterhub/jupyterhub to avoid incorrectly erroring
       - name: check redirects for this PR
-        if: github.repository == 'jupyterhub/jupyterhub'
+        if: ${{ github.event_name == 'pull_request' }}
         run: |
           cd docs
+          export REDIRAFFE_BRANCH=${{ github.base_ref }}
           make rediraffecheckdiff
 
       # this should check currently published 'stable' links for redirects

docs/source/conf.py

@@ -235,7 +235,7 @@ ogp_use_first_image = True
 # If you are basing changes off another branch/ commit, always change back
 # rediraffe_branch to main before pushing your changes upstream.
 #
-rediraffe_branch = os.environ.get("REDIRAFFE_BRANCH", "origin/main")
+rediraffe_branch = os.environ.get("REDIRAFFE_BRANCH", "main")
 rediraffe_redirects = "redirects.txt"
 
 # allow 80% match for autogenerated redirects