Clone the deploy branch before building

As of July 31, 2020, GitHub Pages still fails to deploy if the target
branch contains only one commit (the root commit). By cloning the target
branch before building, we can push the newly built content as a new
commit on top of the existing branch, effectively bypassing this GH
Pages issue.

Also, this preserves the deploy history, though I'm unsure if anyone
really cares.

.github/workflows/docs.yml

@@ -32,6 +32,13 @@ jobs:
           gem update bundler --no-document
           bundle install --path=vendor/bundle --jobs 4 --retry 3
           bundle clean
+      - name: Clone target branch
+        run: |
+          REMOTE_BRANCH="${REMOTE_BRANCH:-gh-pages}"
+          REMOTE_REPO="https://${GITHUB_ACTOR}:${{ secrets.GITHUB_TOKEN }}@github.com/${GITHUB_REPOSITORY}.git"
+
+          echo "Publishing to ${GITHUB_REPOSITORY} on branch ${REMOTE_BRANCH}"
+          git clone --depth=1 --branch="${REMOTE_BRANCH}" --single-branch --no-checkout "${REMOTE_REPO}"
       - name: Build site
         run: bundle exec jekyll build --source docs --destination docs/_site --verbose --trace
         env:
@@ -43,17 +50,11 @@ jobs:
           pushd docs/_site &>/dev/null
           : > .nojekyll
 
-          REMOTE_REPO="https://${GITHUB_ACTOR}:${{ secrets.GITHUB_TOKEN }}@github.com/${GITHUB_REPOSITORY}.git"
-          REMOTE_BRANCH="${REMOTE_BRANCH:-gh-pages}"
-
-          echo "Publishing to ${GITHUB_REPOSITORY} on branch ${REMOTE_BRANCH}"
-          git init --quiet
-          git config user.name "${GITHUB_ACTOR}"
-          git config user.email "${GITHUB_ACTOR}@users.noreply.github.com"
           git add --all
-          git commit --quiet \
+          git -c user.name="${GITHUB_ACTOR}" -c user.email="${GITHUB_ACTOR}@users.noreply.github.com" \
+            commit --quiet \
             --message "Deploy docs from ${GITHUB_SHA}" \
             --message "$SOURCE_COMMIT"
-          git push "$REMOTE_REPO" "+HEAD:${REMOTE_BRANCH}"
+          git push
 
           popd &>/dev/null