CI: Revert to pre-zotero#2520 approach, increase depth to 100

I'm sure there's a better way, but GH Actions are hard to test
locally (the checkout action functions differently in Act! cool!) and
the various supposedly better approaches I've tried have only led
it *always* to fail rather than only failing on old PRs.

.github/workflows/ci.yml

@@ -9,10 +9,12 @@ jobs:
       - uses: actions/checkout@v2
         with:
           submodules: true
-          ref: ${{ github.head_ref }}
 
-      - name: Add PR base ref
-        run: git fetch --depth=1 origin +refs/heads/${{github.base_ref}}:refs/remotes/origin/${{github.base_ref}}
+      # We're in a shallow single-branch clone, so get the origin/master HEAD if not already on master
+      # and get more history on the current branch so we can find the branch point
+      - run: git fetch origin master:master --depth=1
+        if: github.ref != 'refs/heads/master'
+      - run: git fetch --update-shallow --depth=100 origin $(git rev-list HEAD)
 
       - name: Install Node
         uses: actions/setup-node@v1