tests: Fix missing commit in "Upgrade from latest release" workflow

.github/workflows/upgrade-from-latest-release.yml

@@ -60,10 +60,18 @@ jobs:
     - name: Run the backend tests
       run: cd src && npm test
 
+    # Because actions/checkout@v2 is called with "ref: master" and without
+    # "fetch-depth: 0", the local clone does not have the ${GITHUB_SHA} commit.
+    # Fetch ${GITHUB_REF} to get the ${GITHUB_SHA} commit. Note that a plain
+    # "git fetch" only fetches "normal" references (refs/heads/* and
+    # refs/tags/*), and for pull requests none of the normal references include
+    # ${GITHUB_SHA}, so we have to explicitly tell Git to fetch ${GITHUB_REF}.
     - name: Fetch the new Git commits
-      run: git fetch
+      run: git fetch --depth=1 origin "${GITHUB_REF}"
 
     - name: Upgrade to the new Git revision
+      # For pull requests, ${GITHUB_SHA} is the automatically generated merge
+      # commit that merges the PR's source branch to its destination branch.
       run: git checkout "${GITHUB_SHA}"
 
     - name: Install all dependencies and symlink for ep_etherpad-lite